The Safer Affordable Fuel-Efficient (SAFE) Vehicles Rule III for Model Years 2022 to 2031 Passenger Cars and Light Trucks

Federal RegisterDec 5, 2025

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DEPARTMENT OF TRANSPORTATION

National Highway Traffic Safety Administration

49 CFR Parts 523, 531, 533, 536, and 537

[NHTSA-2025-0491]

RIN 2127-AM76

The Safer Affordable Fuel-Efficient (SAFE) Vehicles Rule III for Model Years 2022 to 2031 Passenger Cars and Light Trucks

AGENCY:

National Highway Traffic Safety Administration (NHTSA).

ACTION:

Notice of proposed rulemaking (NPRM).

SUMMARY:

NHTSA, on behalf of the Department of Transportation (DOT), proposes to substantially recalibrate the Corporate Average Fuel Economy (CAFE) program to realign this program with Congressional intent. That recalibration includes proposing to amend DOT's fuel economy standards for light-duty vehicles for model years (MYs) 2022-2026 and MYs 2027-2031. Consistent with statutory requirements, the fuel economy standards proposed in this rule are founded on light-duty vehicles powered by gasoline and diesel fuels, a category that includes non-plug-in hybrid vehicles. In formulating the proposed standards, NHTSA has not considered, consistent with law, the imputed fuel-economy performance of battery-powered electric vehicles (EVs) or the electric operation of vehicles that use plug-in hybrid electric powertrains, nor compliance credits or adjustments to the two-cycle fuel economy test procedures to account for air conditioning and off-cycle technologies. NHTSA also is proposing to eliminate the inter-manufacturer credit trading system and to amend the light-duty vehicle fleet classification system to allocate vehicles into passenger and non-passenger automobile fleets appropriately, based on their attributes and capabilities, starting in MY 2028. Elimination of unlawful considerations, combined with several of the proposed changes, would significantly improve the capabilities of manufacturers to meet fuel economy standards, better align the program with Congressional intent, and reduce manufacturer incentives to design vehicles and add features that are not desired by American consumers and that have questionable real-world fuel economy benefits. NHTSA is therefore proposing to set fuel economy standards that increase from newly proposed MY 2022 standards at a rate of 0.5 percent per year through MY 2026, followed by 0.25 percent per year through MY 2031, with MY 2027 stringency established as a bridge between the two sets of standards. The reduced stringency increases in later years, coupled with a reevaluation of the coefficients that define the functions governing fuel economy standards, are intended to establish maximum feasible standards in a manner that gains real-world fuel-economy-benefits, while enabling the industry to adapt to the proposed substantial recalibration of the CAFE program. NHTSA projects that the amended standards would correspond to the industry fleetwide average for all light-duty vehicles of roughly 34.5 miles per gallon (mpg) in MY 2031.

DATES:

Comments:

Comments are requested on or before January 20, 2026. See the

SUPPLEMENTARY INFORMATION

section on “Public Participation,” below, for more information about written comments. In compliance with the Paperwork Reduction Act, NHTSA is also seeking comments on a modification of an existing information collection. For additional information, see the Paperwork Reduction Act section under Section VIII below. All comments relating to the information collection requirements should be submitted to NHTSA and to the Office of Management and Budget (OMB) at the address listed in the

ADDRESSES

section on or before 45 days from date of publication.

Public Hearings:

NHTSA will hold one virtual public hearing during the public comment period. The agency will announce the specific date and web address for the hearing in a supplemental

Federal Register

notice. The agency will accept oral and written comments on the rulemaking documents and will also accept comments on the Draft Supplemental Environmental Impact Statement (Draft SEIS) at this hearing. The hearing will start at 9 a.m. Eastern time and continue until everyone has had a chance to speak. See the

SUPPLEMENTARY INFORMATION

section on “Public Participation,” below, for more information about the public hearing.

ADDRESSES:

For access to the dockets or to read background documents or comments received, please visit

https://www.regulations.gov,

or Docket Management Facility, M-30, U.S. Department of Transportation, West Building, Ground Floor, Rm. W12-140, 1200 New Jersey Avenue SE, Washington, DC 20590. The Docket Management Facility is open between 9 a.m. and 4 p.m. Eastern time, Monday through Friday, except Federal holidays.

Comments on the proposed information collection requirements should be submitted to: Office of Management and Budget at

www.reginfo.gov/public/do/PRAMain.

To find this information collection, select “Currently under Review—Open for Public Comment” or use the search function. It is requested that comments sent to the OMB also be sent to the NHTSA rulemaking docket identified in the heading of this document.

FOR FURTHER INFORMATION CONTACT:

For technical and policy issues, Joseph Bayer, CAFE Program Division Chief, Office of Rulemaking, National Highway Traffic Safety Administration, 1200 New Jersey Avenue SE, Washington, DC 20590; email:

CAFE_Mbox@dot.gov.

For legal issues, Hannah Fish, NHTSA Office of Chief Counsel, National Highway Traffic Safety Administration, 1200 New Jersey Avenue SE, Washington, DC 20590; email:

CAFE_Mbox@dot.gov.

SUPPLEMENTARY INFORMATION:

Table of Acronyms and Abbreviations

Abbreviation

Term

4WD

Four Wheel Drive.

Air conditioning.

ACME

Adaptive Cylinder Management Engine.

ADEAC

Advanced Cylinder Deactivation.

ADEACD

Advanced cylinder deactivation on a dual-overhead camshaft engine.

ADEACS

Advanced cylinder deactivation on a single overhead camshaft engine.

ADSL

Advanced Diesel Engine.

AEB

Automatic Emergency Braking.

AEO

Annual Energy Outlook.

AER

All-Electric Range.

AERO

Aerodynamic Drag Technology.

AERO0

Base Level Aerodynamic Drag Technology.

AERO5

Aerodynamic Drag, 5% Drag Coefficient Reduction.

AERO10

Aerodynamic Drag, 10% Drag Coefficient Reduction.

AERO15

Aerodynamic Drag, 15% Drag Coefficient Reduction.

AERO20

Aerodynamic Drag, 20% Drag Coefficient Reduction.

AFV

Alternative Fuel Vehicle.

AHSS

Advanced High Strength Steel.

AIS

Abbreviated Injury Scale.

AMFA

Alternative Motor Fuels Act of 1988.

AMPC

Advanced Manufacturing Production Tax Credit.

AMTL

Advanced Mobility Technology Laboratory.

Argonne

Argonne National Laboratory.

ANSI

American National Standards Institute.

APA

Administrative Procedure Act.

Automatic Transmission.

AWD

All-Wheel Drive.

BEV

Battery Electric Vehicle.

BGEPA

Bald and Golden Eagle Protection Act.

BISG

Belt Integrated Starter Generator.

BLS

Bureau of Labor Statistics.

BMEP

Brake Mean Effective Pressure.

BSD

Blind Spot Detection.

BSFC

Brake-Specific Fuel Consumption.

BTW

Brake and Tire Wear.

CAA

Clean Air Act.

CAFE

Corporate Average Fuel Economy.

CARB

California Air Resources Board.

CBI

Confidential Business Information.

CEGR

Cooled Exhaust Gas Recirculation.

CFR

Code of Federal Regulations.

Methane.

CNG

Compressed Natural Gas.

Carbon Dioxide.

COVID-19

Coronavirus disease of 2019.

CPM

Cost Per Mile.

Compression Ratio.

CVC

Clean Vehicle Credits.

CVT

Continuously Variable Transmission.

Curb Weight.

Calendar Year.

CZMA

Coastal Zone Management Act.

DCT

Dual-Clutch Transmission.

DEAC

Dynamic Cylinder Deactivation.

DMC

Direct Manufacturing Costs.

DOE

U.S. Department of Energy.

DOI

U.S. Department of the Interior.

DOHC

Dual-Overhead Camshaft.

DOT

U.S. Department of Transportation.

DSLI

Advanced Diesel Engine With Improvements.

eCVT

Electronic Continuously Variable Transmissions.

EGR

Exhaust Gas Recirculation.

EIA

U.S. Energy Information Administration.

EISA

Energy Independence and Security Act of 2007

E.O.

Executive Order.

EPA

U.S. Environmental Protection Agency.

EPCA

Energy Policy and Conservation Act of 1975.

ESA

Endangered Species Act.

ETDS

Electric Traction Drive System.

Electric Vehicle.

FCEV

Fuel Cell Electric Vehicle.

FCIV

Fuel Consumption Improvement Value.

FCW

Forward Collision Warning.

FEOC

Foreign entity of concern.

FHWA

Federal Highway Administration.

FIP

Federal Implementation Plan.

FRIA

Final Regulatory Impact Analysis.

FTP

Federal Test Procedure.

FWD

Front-wheel Drive.

FWS

U.S. Fish and Wildlife Service.

GCWR

Gross Combined Weight Rating.

GDP

Gross Domestic Product.

GES

General Estimates System.

General Motors.

GREET

Greenhouse gases, Regulated Emissions, and Energy use in Transportation.

GVWR

Gross Vehicle Weight Rating.

HCR

High Compression Ratio.

HCRD

High Compression Ratio Engine with Cylinder Deactivation.

HCRE

High Compression Ratio Engine with Cooled Exhaust Gas Recirculation.

HEG

High Efficiency Gearbox.

HEV

Hybrid Electric Vehicle.

HFET

Highway Fuel Economy Test.

Horsepower.

HVAC

Heating, Ventilation, and Air Conditioning.

IAV

Ingenieurgesellschaft Auto und Verkehr.

ICCT

International Council on Clean Transportation.

ICE

Internal Combustion Engine.

ICR

Information Collection Request.

IIHS

Insurance Institute for Highway Safety.

IRA

Inflation Reduction Act.

LCA

Lane Change Assist.

Light-Duty.

LDW

Lane Departure Warning.

LDWF

Light-Duty Work Factor.

LFP

Lithium Iron Phosphate.

LIVC

Late Intake Valve Closing.

LKA

Lane Keep Assist.

MAD

Minimum Absolute Deviation.

MAGICC

Model for the Assessment of Greenhouse Gas Induced Climate Change.

MBTA

Migratory Bird Treaty Act.

MDPCS

Minimum Domestic Passenger Car Standard.

MDPV

Medium-Duty Passenger Vehicle.

MOVES

Motor Vehicle Emission Simulator.

mpg

Miles Per Gallon.

mph

Miles Per Hour.

Mass Reduction.

MR0

Base Level Mass Reduction Technology.

MSRP

Manufacturer Suggested Retail Price.

Model Year.

NAAQS

National Ambient Air Quality Standards.

NADA

National Automotive Dealers Association.

NAICS

North American Industry Classification System.

NAS

National Academy of Sciences.

NCE

Non-Criteria Emission.

NEMS

National Energy Modeling System.

NEPA

National Environmental Policy Act.

NHPA

National Historic Preservation Act.

NHTSA

National Highway Traffic Safety Administration.

NMC

Nickel Manganese Cobalt.

Nitrogen Oxide.

NPRM

Notice of Proposed Rulemaking.

NRC

National Research Council.

NTTAA

National Technology Transfer and Advancement Act.

NVO

Negative Valve Overlaps.

gpm

gallons per mile.

Off-Cycle.

OCR

Optical Character Recognition.

OEM

Original Equipment Manufacturer.

OHV

Overhead Valve.

OLS

Ordinary Least Square.

OMB

Office of Management and Budget.

OPEC

Organization of the Petroleum Exporting Countries.

ORNL

Oak Ridge National Laboratory.

PAEB

Pedestrian Automatic Emergency Braking.

Passenger Car.

PEF

Petroleum Equivalency Factor.

PHEV

Plug-in Hybrid Electric Vehicle.

2.5

Particulate matter 2.5 microns or less in diameter.

PPC

Passive Prechamber Combustion.

ppm

parts per million.

PRA

Paperwork Reduction Act of 1995.

PRIA

Preliminary Regulatory Impact Analysis.

ROLL

Tire Rolling Resistance.

ROLL0

Base Level Tire Rolling Resistance.

ROLL10

Tire Rolling Resistance, 10% Improvement.

ROLL20

Tire Rolling Resistance, 20% Improvement.

ROLL30

Tire Rolling Resistance, 30% Improvement.

RPE

Retail Price Equivalent.

RPM

Revolutions Per Minute.

RRC

Rolling Resistance Coefficient.

RWD

Rear-Wheel Drive.

SAE

Society of Automotive Engineers.

SEC

Securities and Exchange Commission.

SEIS

Supplemental Environmental Impact Statement.

SGDI

Stoichiometric Gasoline Direct Injection.

SHEV

Strong Hybrid Electric Vehicle.

SHEVPS

Power-Split Strong Hybrid Electric Vehicle.

Spark Ignition.

SIP

State Implementation Plan.

SKIP

Refers to skip input in Market Data Input File.

SOC

State of Charge.

SOHC

Single Overhead Camshaft.

Sulfur Oxide.

SS12V

12V Micro Hybrid Start-Stop System.

SUV

Sport Utility Vehicle.

SwRI

Southwest Research Institute.

TAR

Technical Assessment Report.

TS&D

Fuel Transportation, Storage, and Distribution.

TSD

Technical Support Document.

TURBO0

Reference baseline turbocharged downsized technology.

TURBO1

Turbocharged downsized technology.

TURBO2

Advanced turbocharged downsized technology.

TURBOAD

Turbocharged engine with advanced cylinder deactivation.

TURBOD

Turbocharged engine with cylinder deactivation.

TURBOE

Turbocharged engine with cooled exhausted recirculation.

UMRA

Unfunded Mandates Reform Act.

U.S.

United States.

U.S.C

Unites States Code.

VCR

Variable Compression Ratio.

Volpe or Volpe Center

Volpe National Transportation Systems Center.

VMT

Vehicle Miles Traveled.

VSL

Value of a Statistical Life.

VTG

Variable Turbo Geometry.

VTGE

Variable Turbo Geometry (Electric).

VVL

Variable Valve Lift.

VVT

Variable Valve Timing.

VWA

Volkswagen Group of America.

ZEV

Zero Emission Vehicle.

Does this action apply to me?

This proposal affects companies that manufacture or sell new passenger automobiles (passenger cars) and non-passenger automobiles (light trucks), as defined under NHTSA's CAFE regulations.

Regulated categories and entities include:

See

49 CFR part 523.

EP05DE25.007

This list is not intended to be exhaustive but rather provides a guide regarding entities likely to be regulated by this action. To determine whether particular activities may be regulated by this action, you should carefully examine the regulations. You may direct questions regarding the applicability of this action to the persons listed in

FOR FURTHER INFORMATION CONTACT

Table of Contents

I. Executive Summary

II. Technical Foundation for the NPRM Analysis

A. Why is NHTSA conducting this analysis?

1. What are the key components of NHTSA's analysis?

2. How do statutory requirements shape NHTSA's analysis?

3. What updated capabilities and assumptions does the current model reflect as compared to the version used in the analysis of the 2024 final rule?

B. What is NHTSA analyzing?

C. What inputs does the compliance analysis require?

1. What inputs does the analysis require for 2022-2026?

2. What inputs does the compliance analysis require for 2027-2031?

a. Technology Options and Pathways

b. Defining Manufacturers' Current Technology Positions in the Analysis Fleet

c. Technology Effectiveness Values

d. Technology Costs

e. Simulating Tax Credits

f. Technology Applicability Equations and Rules

D. Technology Pathways, Effectiveness, and Cost

1. Engine Paths

2. Transmission Paths

3. Hybridization Paths

4. Road Load Reduction Paths

5. Mass Reduction

6. Aerodynamic Improvements

7. Low Rolling Resistance Tires

8. Simulating Air-Conditioning Efficiency and Off-Cycle Technologies

E. Consumer Responses to Manufacturer Compliance Strategies

1. Consumer Responses to Manufacturer Compliance Strategies for 2027-2031

a. Macroeconomic and Consumer Behavior Assumptions

b. Fleet Composition

(1) Sales

(2) Scrappage

c. Changes in Vehicle-Miles Traveled

d. Changes to Fuel Consumption

F. Simulating Emissions Impacts of Regulatory Alternatives

G. Simulating Economic Impacts of Regulatory Alternatives

1. Private Costs and Benefits

2. External Costs and Benefits

H. Simulating Safety Effects of Regulatory Alternatives

1. Mass Reduction Impacts

2. Sales/Scrappage Impacts

3. Rebound Effect Impacts

4. Value of Safety Impacts

III. Regulatory Alternatives Considered in This NPRM

A. General Basis for Alternatives Considered

1. MYs 2022-2026

2. MYs 2027-2031

3. Minimum Domestic Passenger Car Standard Analysis Update

B. Regulatory Alternatives Considered

1. No-Action Alternatives for Passenger Cars and Light Trucks

a. No-Action Alternative for MYs 2022-2026 Amendment

b. No-Action Alternative for MYs 2027-2031 Amendment

2. Action Alternatives for Passenger Cars and Light Trucks

a. Action Alternatives for MYs 2022-2026 Amendment

(1) Alternative 1

(2) Alternative 2—Preferred Alternative

(3) Alternative 3

b. Action Alternatives for MYs 2027-2031 Amendment

(1) Alternative 1

(2) Alternative 2—Preferred Alternative

(3) Alternative 3

IV. Effects of the Regulatory Alternatives

A. Effects of the Regulatory Alternatives for MYs 2022-2026

B. Effects of the Regulatory Alternatives for 2027-2031

1. Effects on Vehicle Manufacturers

2. Effects on Society

3. Physical and Environmental Effects

4. Sensitivity Analysis

V. Basis for NHTSA's Tentative Conclusion That the Proposed Standards Are Maximum Feasible

A. EPCA, as Amended by EISA

1. Administrative Provisions Governing CAFE Standard Setting

a. Lead Time, Amendatory Authority, and Number of Model Years for Which Standards May Be Set at a Time

b. Separate Standards for Passenger Automobiles and Non-Passenger Automobiles

c. Minimum Standards for Domestic Passenger Automobiles

d. Attribute-Based Standards Defined by a Mathematical Function

2. Maximum Feasible Standards

a. Technological Feasibility

b. Economic Practicability

c. The Effect of Other Motor Vehicle Standards of the Government on Fuel Economy

d. The Need of the United States to Conserve Energy

(1) Consumer Costs and Fuel Prices

(2) National Balance of Payments

(3) Environmental Effects

(4) Foreign Policy Implications

e. Factors That NHTSA Is Prohibited From Considering

f. Additional Considerations Relevant to NHTSA's Statutory Determination of Maximum Feasibility

B. Other Statutory Requirements

1. Administrative Procedure Act

2. National Environmental Policy Act

C. Evaluating the Statutory Factors and Other Considerations to Arrive at the Proposed Standards

1. Why is NHTSA's tentative conclusion different from the 2020, 2022, and 2024 final rules?

2. Considerations Justifying the Proposed Standards

a. Technological Feasibility and the Effect of Other Motor Vehicle Standards of the Government on Fuel Economy

b. Economic Practicability and Safety (Both Independently and as a Subset of Economic Practicability)

c. The Need of the United States To Conserve Energy

3. Draft Supplemental Environmental Impact Statement Analysis Results

D. Severability

VI. Compliance and Enforcement

A. Background and Overview of Compliance and Enforcement

B. Proposed Changes to the CAFE Program

1. Modification of Vehicle Classification in the CAFE Program

a. Non-Passenger Automobile Definition

b. Proposed Changes to Criteria for Off-Highway Capability

c. Proposed Changes to Criteria for Functional Performance

(1) Automobiles With Three or More Rows of Seating

(2) Light-Duty Work Factor

2. Removal of Credit Trading in the CAFE Program

3. Technical Amendments To Remove References to EPA's Regulations for AC Efficiency and Off-Cycle Fuel Consumption Improvement Values

4. Modification of Manufacturer Reporting Requirements

C. Technical Amendments

1. Technical Amendments To Remove Residual Mention of Fuel Efficiency Standards for Trailers in NHTSA's Vehicle Classification Regulations

2. Technical Amendment To Remove Heavy-Duty Trailers From the List of Heavy-Duty Vehicle Regulatory Categories

3. Technical Amendments To Remove Civil Penalties for Non-Compliance With Fuel Economy Standards From the CAFE Program

4. Additional Technical Amendments

a. Technical Amendments to Part 523

b. Technical Amendments to Part 531

c. Technical Amendments to Part 533

d. Technical Amendments to Part 536

e. Technical Amendments to Part 537

VII. Public Participation

VIII. Regulatory Notices and Analyses

A. Executive Order 12866, “Regulatory Planning and Review”; Executive Order 13563, “Improving Regulation and Regulatory Review”; Executive Order 14192, “Unleashing Prosperity Through Deregulation”; and Executive Order 14219, “Ensuring Lawful Governance and Implementing the President's `Department of Government Efficiency' Deregulatory Initiative”

B. Environmental Considerations

1. National Environmental Policy Act

2. Clean Air Act as Applied to NHTSA's Proposed Rule

3. Endangered Species Act (ESA)

4. Other Regulatory Analyses Discussed in the Draft SEIS

5. Executive Order 13045: “Protection of Children From Environmental Health Risks and Safety Risks”

6. Executive Order 14154: “Unleashing American Energy”

7. Executive Order 14173: “Ending Illegal Discrimination and Restoring Merit-Based Opportunity”

C. Regulatory Flexibility Act

D. Executive Order 13132 (“Federalism”)

E. Executive Order 12988 (“Civil Justice Reform”)

F. Executive Order 13175 (“Consultation and Coordination With Indian Tribal Governments”)

G. Unfunded Mandates Reform Act

H. Regulation Identifier Number

I. National Technology Transfer and Advancement Act

J. Department of Energy Review

K. Paperwork Reduction Act

L. Rulemaking Summary, 5 U.S.C. 553(b)(4)

IX. Regulatory Text

I. Executive Summary

The relationship between the light-duty vehicle market and the CAFE program has gone through several cycles over its almost 50-year history. First created to require conservation of petroleum in response to price shocks caused by the Arab oil embargoes of the 1970s, the CAFE program has led not only to the desired improvements in fuel economy but also created unintended responses from vehicle manufacturers—often to the detriment of consumers.

Over the CAFE program's history, separate standards for the passenger car and light truck fleets (referred to by law as passenger automobiles and non-passenger automobiles) have led manufacturers to reshape the market in unanticipated ways—such as by almost eliminating the production of station wagons (passenger cars that generally have more robust cargo capacity, adding mass and reducing fuel economy) in favor of vehicles like minivans and crossover utility vehicles (considered light trucks, and subject to less stringent standards).

Strict mile-per-gallon-based standards in the program's early years also led manufacturers to seek significant reductions in vehicle size and mass, leading to increased injury or fatality risk for occupants of smaller vehicles involved in a crash.

NHTSA sought to mitigate these responses by creating attribute-based standards that relate the “footprint” size of vehicles to fuel economy, to some positive effect.

Transportation Research Board and National Research Council, Effectiveness and Impact of Corporate Average Fuel Economy (CAFE) Standards, National Academies Press: Washington, DC (2002), available at:

https://nap.nationalacademies.org/catalog/10172/effectiveness-and-impact-of-corporate-average-fuel-economy-cafe-standards

(accessed: Feb. 7, 2024). This report describes at length and quantifies the potential safety problem with average fuel economy standards that specify a single numerical requirement for the entire industry, noting that smaller and lighter vehicles incentivized by those standards could be less safe for their occupants.

Meanwhile, the U.S. Environmental Protection Agency (EPA) started providing special fuel economy adjustments for technologies that had potential for fuel economy improvements but were not measurable using the laboratory test procedures (

i.e.,

the “two-cycle” tests) for vehicle fuel economy. This included accommodating adjustments to efficiency values if manufacturers implemented preferred air conditioning (AC) technologies, and if manufacturers installed special technologies with purported fuel-saving benefits that could not be captured on the aforementioned two-cycle tests, accordingly known as “off-cycle” (OC) technologies (

e.g.,

vehicle stop/start functions that shut off the engine when the vehicle has stopped). These regulatory adjustments have led to widespread adoption of technologies with uncertain real-world benefits, added costs, and, in many cases, consumer backlash.

The creation of a system for inter-manufacturer credit trading—intended to improve the cost-effectiveness of the CAFE program by allowing manufacturers that could improve the fuel economy of their fleets more cost-effectively to earn credits for exceeding fuel economy standards and sell those credits to manufacturers that would need to incur higher costs to meet fuel economy standards—has also resulted in a windfall for EV-exclusive manufacturers that sell credits to other non-EV manufacturers, which in turn pay for those credits with capital that could be invested toward improving the fuel economy performance or other desirable attributes of their traditional fleets. The enormous fuel economy values assigned to EVs have, heretofore, been included in the baseline fleet fuel economy for subsequent CAFE rulemakings upon which stringency increases are applied—thereby significantly increasing the fuel economy requirements for traditional gasoline- or diesel-fueled fleets.

In a hypothetical and simplified example, if the baseline passenger car fleet of vehicles with an identical footprint consisted of nine gasoline-powered vehicles achieving 30 mpg and one EV achieving 150 mpg, the baseline fleet to which stringency increases would apply would be measured at 42 mpg. When CAFE standards are set unlawfully considering EV fuel economy, manufacturers of gasoline-powered vehicles would face a challenge in catching up to the overall fleet fuel economy, requiring disproportionate investment in fuel-saving technologies, and incentivizing the purchase of regulatory credits from the EV manufacturer.

At the same time, the classification system that has long divided the fleet between passenger cars (intended to

move passengers) and light trucks (intended to move cargo or operate off road) no longer lives up to its anticipated use. Indeed, while 68 percent of the light-duty fleet meets the current light truck regulatory definition, the majority of these vehicles (

e.g.,

all-wheel drive (AWD) crossover utility vehicles, vehicles with three or more rows of seating, and vehicles that do not have an approach angle high enough to handle an off-highway obstacle) cannot realistically operate off road and have little value moving cargo. Instead, most of these vehicles are designed and intended primarily to move passengers but have additional features solely to meet regulatory definitions

—resulting in little added functionality, reduced fuel economy performance, added cost, and a fairly homogenous design language lacking in creativity.

Section VI discusses NHTSA's proposal to amend regulatory definitions for passenger and non-passenger automobiles in detail and includes examples of manufacturers excluding or including specific features solely to meet regulatory definitions. Two examples discussed in more detail in Section VI include manufacturers discontinuing FWD versions of vehicles after NHTSA properly reclassified over 1 million FWD automobiles as passenger automobiles in line with EPCA and opting to instead manufacture only AWD or 4WD versions to keep more of their products in the non-passenger automobile fleets (74 FR 14196, Mar. 30, 2009), and manufacturers including aerodynamic technologies to increase on-highway functionality instead of opting to meet approach angle requirements, which would make the vehicle more capable of approaching off-highway obstacles and, thus, more off-highway capable.

While the CAFE program was intended to push manufacturers to improve fuel economy while preserving their ability to design and produce vehicles that meet market demands, the system has spun off its axis and requires recalibration. Instead of allowing manufacturers to design and produce vehicles they believe their customers will want and need, while spreading real-world fuel economy improvements across their fleets, the system has increasingly led manufacturers to try to fit square vehicle pegs in round classification holes to force the adoption of technologies that do not meet the demands of American families simply to obtain on-paper fuel economy improvements that may have little basis in reality. All of this adds inefficiency and cost—pushing even more consumers out of an already unaffordable new car market.

By delegation of authority from the Secretary of Transportation (the Secretary), NHTSA is proposing to amend the previously promulgated CAFE standards applicable to passenger and non-passenger automobiles (colloquially referred to as passenger cars and light trucks, and together known as light-duty vehicles) produced for MYs 2022-2026 and MYs 2027-2031. Proposing amended standards beginning with MY 2022 is consistent with the Secretary's direction in the January 28, 2025, memorandum titled “Fixing the CAFE Program” and is also the earliest model year for which NHTSA has not concluded CAFE compliance proceedings; additional discussion regarding NHTSA's proposal to amend standards beginning in MY 2022 can be found in Section V.

Consistent with the terms of the CAFE program mandated in the Energy Policy and Conservation Act (EPCA), as amended by the Energy Independence and Security Act (EISA) and other laws (codified in chapter 329 of title 49, United States Code), the fuel economy standards proposed herein are founded on light-duty vehicles powered by gasoline and diesel fuels, a category that includes non-plug-in hybrid vehicles.

In formulating the proposed standards, NHTSA has not considered the imputed fuel-economy performance of EVs or the electric operation of plug-in hybrid electric vehicles (PHEVs). This approach marks a change from previous rulemakings, as described above, but brings the CAFE program into compliance with statutory restrictions.

Non-plug-in hybrid vehicles are not dual-fueled vehicles under Chapter 329 because any electricity generated by the electric motors or other electric components are generated solely by the petroleum-fueled engine and the batteries are incapable of charging from an external source: “a vehicle which is entirely dependent on a petroleum fuel for its motive power, regardless of whether electricity is used in the powertrain, is powered by petroleum.” 63 FR 66066 (Dec. 1, 1998).

This proposed rule fulfills NHTSA's statutory obligation to set CAFE standards at the maximum feasible level that the agency determines vehicle manufacturers can achieve in each model year, balancing four key factors: technological feasibility, economic practicability, the need of the Nation to conserve energy, and the effect of other Federal regulations on fuel economy.

This balancing must take into account current and projected circumstances and cannot consider the availability of alternative fuel technologies (

e.g.,

EVs or PHEV electric operation), or compliance credits.

This action is also consistent with Executive Order (E.O.) 14148, “Initial Rescissions of Harmful Executive Orders and Actions,”

and E.O. 14154, “Unleashing American Energy,”

as well as the Secretarial memorandum titled “Fixing the CAFE Program.”

49 U.S.C. 32902(a) and (f).

49 U.S.C. 32902(h).

90 FR 8237 (Jan. 28, 2025).

90 FR 8353 (Jan. 29, 2025).

See DOT, Memorandum: Fixing the CAFE Program (2025), available at:

https://www.transportation.gov/briefing-room/memorandum-fixing-cafe-program

(accessed: Sept. 10, 2025).

The standards presented in this proposal significantly differ from those finalized in the 2020, 2022, and 2024 rules because, in formulating those prior standards, NHTSA considered both the fuel economy of EVs and PHEVs and compliance credits that could be earned when a manufacturer over-complied with an applicable fuel economy standard impermissibly. As a result, the fuel economy standards previously established by NHTSA for passenger cars and light trucks for MYs 2022-2031 failed to satisfy substantive statutory requirements. NHTSA is proposing in this NPRM the “maximum feasible” amended fuel economy requirements for the model years in question that best reflect and balance the various practical considerations and limitations mandated for the CAFE program.

This rulemaking is intended to establish maximum feasible fuel economy standards while restoring the functionality intended by Congress. It marks a significant reset. As an initial matter, NHTSA proposes to remove consideration of prohibited technologies and credits from every aspect of the standards development process to bring the program back within its statutory constraints. NHTSA discussed extensively its prior unlawful consideration of prohibited technologies and credits in the standards development process in the final rule,

Resetting the Corporate Average Fuel Economy Program,

and includes a more detailed discussion in Section V, below.

90 FR 24518 (June 11, 2025).

NHTSA is proposing to remove consideration of AC efficiency and OC fuel consumption improvement values (FCIVs) from its standard-setting analysis starting with MY 2028, which is the first year in which a removal of FCIVs could go into effect.

This change will ensure that NHTSA's CAFE standards are achievable without the implementation of technologies not demanded by consumers and with questionable fuel economy benefits.

49 U.S.C. 32904(d).

The agency also proposes to eliminate the inter-manufacturer credit trading program (which is authorized, but not required, by 49 U.S.C. 32903(f)) beginning with MY 2028. This change in the program is long overdue. While NHTSA does not consider the availability of credits or credit trading in

establishing standards, the agency believes that eliminating inter-manufacturer credit trading will encourage manufacturers to provide for steady improvement in fuel economy across their fleets over time, as opposed to relying upon credits acquired from third-party EV manufacturers. NHTSA recognizes that manufacturers have made investments in particular compliance pathways—pathways that may include purchasing credits from other manufacturers even though the availability of those credits is uncertain—and is proposing this change beginning with MY 2028 to provide manufacturers with adequate transition time, in recognition of any particular reliance interests in the trading program to achieve compliance, before the program ends. However, NHTSA is proposing standards in this notice at levels that do not consider the use of compliance credits, thus minimizing any impacts that this change may have on manufacturers' decisions about compliance pathways. Moreover, this change will not impact automakers' ability to

transfer

earned credits between different categories of vehicles in their own fleets or carry their own credits forwards and backwards across model years, as prescribed by statute.

The agency also proposes a substantial reclassification of the light-duty fleet in a manner intended by Congress in creating the CAFE program—with the passenger car fleet consisting of vehicles primarily designed to move people, and the light truck fleet consisting of vehicles primarily designed to operate off road or move cargo. NHTSA believes these proposed changes are necessary to restore the CAFE program to its intended orbit but recognizes the changes will introduce significant design consideration for manufacturers. Moving a large fraction of vehicles previously classified as light trucks into a manufacturer's passenger vehicle fleet will have a significant effect on the overall fuel economy performance of the manufacturer's passenger fleet—after all, even if based upon the same platform as a passenger car, the additional vehicle height adds significant mass and decreases fuel economy. Meanwhile, removal of vehicles from a manufacturer's light truck fleet will leave that fleet consisting of even heavier and less aerodynamic vehicles, such as large sports utility vehicles and pickup trucks, thereby decreasing the overall average fuel economy of the light truck fleet. Accordingly, while a manufacturer's combined overall fleet fuel economy may remain the same, both its passenger car and light truck fleets will necessarily achieve lower measured fuel economy. NHTSA is also proposing to update the classification criteria from technology-based to performance-based standards where applicable, consistent with best practices for regulation. This proposal intends to take these changes into account through amendments to both the footprint curves and standards applicable to various points within the curves. NHTSA intends that, as a result of this proposed update, automobiles classified as non-passenger will exhibit true non-passenger capabilities that display relevant off-highway vehicle attributes such as approach angle and running clearance or include design features that provide higher payload and towing abilities for transporting property.

By surveying the measured fuel economy performance of gasoline- and diesel-powered passenger cars and light trucks produced for the U.S. market in MY 2022, NHTSA has created a maximum feasible foundation from which to establish standards for subsequent model years. NHTSA is proposing to set fuel economy standards that increase from the newly proposed MY 2022 standards at a rate of 0.5 percent per year through MY 2026 followed by 0.25 percent per year through MY 2031, with MY 2027 stringency as a bridge between the two sets of standards.

In addition to the proposed standards (also referred to as the “Preferred Alternative”) NHTSA considers a range of regulatory alternatives for each fleet, consistent with the agency's obligations under the Administrative Procedure Act (APA), National Environmental Policy Act (NEPA), and E.O. 12866. The regulatory alternatives are as follows:

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NHTSA

has concluded tentatively that the levels of standards represented by Alternative 2 are the maximum feasible level for these model years, as discussed in more detail in Section V of this preamble. NHTSA has determined that the proposed standards satisfy the statutory requirements of maximum feasibility across the full range of gasoline- and diesel-powered vehicles currently on the market. These standards will be appropriately stringent in promoting fuel efficiency in the Nation's light-duty vehicle fleet while remaining technologically feasible and economically practicable to achieve without regard to EV dedicated fuel economy or PHEV electric operation. The proposed standards also consider the effect of other Federal regulatory mandates on the fuel economy performance of new motor vehicles, as well as the need of the Nation to conserve energy. NHTSA has tentatively determined that it is both reasonable and congruent with EPCA's energy conservation goals to weigh the need of the United States to conserve energy such that vehicle fuel economy standards require continuous improvements over time, but at sustainable levels for manufacturers, consumers, and society at large. In particular, the diminishing effects attributable to fuel economy improvements from higher standards moderates against weighing the need of the United States to conserve energy too heavily compared to the other statutory factors.

Manufacturers have limited supplies of capital for technological advancement and are constrained in recovering those investments by what consumers can afford to pay for technological innovations in new vehicles. Maximum feasible fuel economy standards, when set appropriately weighing economic practicability, should never incentivize manufacturers to add technology that consumers reject at the cost of investments in, or application of, for instance, vehicle safety technologies. Instead, when truly maximum feasible standards apply, manufacturers should be able continually to develop, and apply, both proven fuel-saving and safety-enhancing technologies in such a manner that allows consumers both to desire and to afford the new vehicle.

Percentages in the table represent the year over year reduction in gal/mile applied to the mpg values on the target curves. The reduction in gal/mile results in an increased mpg.

As an example, a vehicle owner who drives a light vehicle 15,000 miles per year and trades in a vehicle with fuel economy of 15 mpg for one with fuel economy of 20 mpg, will reduce their annual fuel consumption from 1,000 gallons to 750 gallons—saving 250 gallons annually. If, however, that owner trades in a vehicle with fuel economy of 30 mpg for one with fuel economy of 40 mpg, then the owner's annual gasoline consumption would drop from 500 gallons/year to 375 gallons/year—a fuel savings of only 125 gallons even though the mpg improvement is twice as large. Going from 40 to 50 mpg would save only 75 gallons/year. Yet each additional fuel economy improvement becomes much more expensive as the easiest to achieve low-cost technological improvement options are exhausted.

NHTSA's preliminary conclusion is that this decision best comports with statutory requirements and is justified to reset standards set in final rules issued in 2020, 2022, and 2024, respectively, which were established improperly above the maximum feasible level because NHTSA considered statutorily prohibited factors in establishing those

standards.

Those rules resulted in distortions in the marketplace, which this proposed rule would minimize. These distortions include major non-market-based changes in automobile designs and the introduction of fundamental alterations in their production processes not primarily driven by market demand.

85 FR 24174 (Apr. 30, 2020); 87 FR 25710 (May 2, 2022); 89 FR 52540 (June 24, 2024).

Increasing the stringency of standards at modest annual rates, following a reset to eliminate the consideration of impermissible factors that were applied in setting the current standards, and coupled with a re-examination of the shape of the fuel economy target functions and the vehicle classification definitions, best comports with statutory requirements. Moreover, the level, shape, and applicability of the standards to the proposed passenger and non-passenger automobile fleets are justified by the inappropriate distortions the existing regulations have caused in the marketplace. Those regulations resulted in unnecessary regulatory burdens that did not further statutory purposes because the standards were not attainable for the gasoline- and diesel-powered vehicle fleet.

The proposed CAFE standards remain vehicle-footprint-based, like the current CAFE standards in effect since MY 2011. The footprint of a vehicle is the area calculated by multiplying the wheelbase times the track width, essentially the rectangular area of a vehicle measured from tire to tire where the tires hit the ground. This means that the standards are defined by mathematical equations that represent constrained linear functions relating vehicle footprint to fuel economy targets for passenger cars and light trucks.

For this proposal, NHTSA has updated the mathematical functions (

i.e.,

the target curves relating footprint to fuel economy) for passenger cars and light trucks based on the latest available data. NHTSA has concluded preliminarily, based on this data, that the relationship between footprint and fuel economy has shifted from MY 2008 (the model year on which the current curves are based) and it is thus appropriate to modify the mathematical functions accordingly. NHTSA has also updated the functions that would be applied beginning in MY 2028 to reflect changes based on the proposed reclassified fleet.

Generally, passenger cars have more stringent targets than light trucks regardless of footprint, and smaller vehicles will have more stringent targets than larger vehicles because smaller vehicles are generally more fuel efficient. No individual vehicle or vehicle model need meet its target exactly, but a manufacturer's compliance is determined by how its average fleet fuel economy compares to the average fuel economy of the targets of the vehicles it manufactures.

NHTSA estimates that the proposed standards would correspond to a combined industry fleetwide average of roughly 34.5 mpg in MY 2031 for passenger cars and light trucks.

NHTSA notes that this is a projection, since the actual CAFE standards are the footprint target curves for passenger cars and light trucks. This is important because it means that the ultimate fleetwide levels will vary depending on the mix of vehicles that manufacturers produce for sale in those model years. NHTSA also calculates and presents “estimated achieved” fuel economy levels, which differ somewhat from the estimated required levels for each fleet, for each year.

Note that the industry-average required and achieved values presented below reflect the end of manufacturers' ability to claim AC and FCIV adjustments, beginning in MY 2028, and updated vehicle classification regulatory definitions, which are also applicable beginning in MY 2028.

NHTSA notes both that real-world fuel economy is generally 20-30 percent lower than the estimated required CAFE level stated above, since CAFE compliance is evaluated per 49 U.S.C. 32904(c) Testing and Calculation Procedures, which states that the EPA Administrator (responsible under EPCA/EISA for measuring vehicle fuel economy) must use the same procedures used for MY 1975 (weighted 55 percent urban cycle and 45 percent highway cycle) or comparable procedures. Colloquially, this is known as the 2-cycle test. The “real-world” or 5-cycle evaluation includes the 2-cycle tests and three additional tests that are used to adjust the city, and highway estimates to account for higher speeds, AC use, and colder temperatures. In addition to calculating vehicle fuel economy, EPA is responsible for providing the fuel economy data that is used on the fuel economy label on all new cars and light trucks, which uses the “real-world” values. In 2006, EPA revised the test methods used to determine fuel economy estimates (city and highway) appearing on the fuel economy label of all new cars and light trucks sold in the United States, effective with MY 2008 vehicles.

NHTSA's analysis reflects that almost all manufacturers make the technological improvements prompted by CAFE standards at times that coincide with existing product “refresh” and “redesign” cycles, rather than unrealistically applying new technology every year regardless of those cycles. It is significantly more cost effective to make fuel economy-improving technology updates when a vehicle is being updated. See the Draft TSD and preamble Section II for additional discussion about manufacturer refresh and redesign cycles.

For simplification, NHTSA provides industry-wide mpg estimates corresponding to the proposed standards in the table below but reiterates that the coefficients that define the mathematical functions comprise the actual standards.

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To the

extent that manufacturers appear to be over-complying with required fuel economy levels in MY 2027, NHTSA notes that this is due to factors including previous application of fuel economy technologies required by standards set improperly for prior model years that unlawfully considered prohibited alternative fuel (

e.g.,

EV) technology applications. Once the program is restored to its intended strictures and standards are established that consider all statutory factors and limitations appropriately, manufacturers that previously applied technologies to meet exaggerated requirements will have relief, while manufacturers that faced certain penalties can continue to improve efficiency to meet maximum feasible standards. NHTSA's review of achieved compliance at the manufacturer level also shows that, while some manufacturers manage to achieve greater over-compliance, other manufacturers are expected to achieve compliance values that will track the levels of the new standards more closely. In addition, NHTSA believes that the proposed standards established for model years prior to the significant MY 2028 fleet reclassification will allow manufacturers to plan strategically with sufficient lead time to manage that transition within their projected model year sales cycles. For all fleets, average requirements and average achieved CAFE levels will depend ultimately on manufacturer and consumer response to standards, technology developments, economic conditions, fuel prices, and other factors.

There is no legal requirement for combined passenger car and light truck fleets, but NHTSA presents information this way in recognition of the fact that many readers will be accustomed to seeing such a value.

NHTSA is also proposing new minimum domestic passenger car CAFE standards (MDPCS) for MYs 2022-2026 and MYs 2027-2031 as required by EISA, which are applied to passenger cars that are deemed to be manufactured in the United States. Section 32902(b)(4) of 49 U.S.C. requires NHTSA to project the minimum domestic standard when it promulgates passenger car standards for a model year; these standards are shown in Table I-3 below. NHTSA continues to apply an offset (albeit a far smaller one than was first used in the 2020 final rule and applied to the 2022 and 2024 final rules) when calculating the MDPCSs for MYs 2027-2031, reflecting prior differences between passenger car footprints forecast originally by the agency and passenger car footprints as they occurred in the real world. The proposed minimum domestic passenger car standards (MDPCS) for each model year are as shown in the table below.

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NHTSA uses the CAFE Compliance and Effects Modeling System (the CAFE Model) developed and maintained by the Volpe National Transportation Systems Center (Volpe Center or Volpe) as a tool for assessing the likely regulatory effects of the proposal and various regulatory alternatives. The Model does not determine which standards satisfy the requirements of EPCA, and no model can predict precisely the engineering configurations automakers are likely to introduce in response to evolving trends in market demand. However, the analysis developed using the CAFE Model provides further support for NHTSA's preliminary judgment that the standards proposed in this rule are the maximum standards that are technologically feasible and economically practicable for the gasoline- and diesel-powered vehicles covered by the proposed rule, considering the effect of other motor vehicle standards of the Government on fuel economy, and the need of the United States to conserve energy.

One significant modification from previous standard-setting proceedings and previous applications of the CAFE Model is that NHTSA did not include EVs in the base fleet for analysis purposes and did not consider or model the potential production of EVs as a CAFE compliance strategy for automakers. Section 32902 of chapter 49 directs NHTSA to establish fuel economy standards that are feasible and practicable for gasoline- and diesel-powered vehicles without regard to any reliance on non-gasoline- or diesel-powered alternatives. Automakers, of course, are free to produce EVs in response to market demand, and their production and sale of EVs will earn credit toward compliance with the CAFE standards in accordance with the “petroleum equivalency factor,” or “PEF,” prescribed by the Department of Energy (DOE).

49 U.S.C. 32904(a)(2)(B); Public Law 96-185, 93 Stat. 1324 (1980).

https://www.congress.gov/96/statute/STATUTE-93/STATUTE-93-Pg1324.pdf

; 10 CFR part 474.

Additional updates to the CAFE Model and its inputs since the 2024 final rule include updating the Market Data Input File to reflect the change in analysis fleet from MYs 2022-2024, updating the modeling capability to allow for vehicle reclassification, updating the Scenarios Input File to set the value of civil penalties at zero,

updating the Parameters Input File to set the monetary value of changes in non-criteria emissions at zero, updating other economic values, such as rebound elasticity and the payback periods, and updating fuel price projections using the 2025 Annual Energy Outlook's (AEO) Alternative Transportation Case. These and other updates are described in more detail in Section II and the Draft TSD.

See

Public Law 119-21, 139 Stat. 72 (July 4, 2025).

https://www.congress.gov/119/plaws/publ21/PLAW-119publ21.pdf.

NHTSA estimates that this proposed rule would reduce the average up-front vehicle costs due to CAFE standards by approximately $900, cutting in half what consumers might expect to pay as a result of increased requirements under the No-Action Alternative.

NHTSA also estimates that this rule will be net beneficial economically for society. The tables below summarize estimates of selected impacts viewed from both the MY and calendar year (CY) perspectives,

for each of the regulatory alternatives, relative to the No-Action Alternative.

The bulk of the analysis for passenger cars and light trucks presents a “model year” perspective rather than a “calendar year” perspective. The model year perspective considers the lifetime impacts attributable to all passenger cars and light trucks produced through MY 2031, accounting for the operation of these vehicles over their entire lives (with some MY 2031 vehicles estimated to be in service as late as 2050). This approach emphasizes the role of the model years for which new standards are being proposed. The calendar year perspective, on the other hand, includes the annual impacts attributable to all vehicles estimated to be in service in each calendar year for which the analysis includes a representation of the entire registered light-duty fleet. For this proposed rule, this calendar year perspective covers each of CYs 2024-2050. Compared to the model year perspective, the calendar year perspective includes model years of vehicles produced in the longer term, beyond those model years for which standards are being proposed.

For this and similar tables in this section, net benefits may differ from benefits minus costs due to rounding.

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The current estimates of costs and benefits are important considerations, performed as directed by E.O. 12866, and also serve as an informative data point in NHTSA's consideration of the factors that NHTSA is required to balance by statute when determining maximum feasible standards. NHTSA concludes, for the purposes of this proposal, that Alternative 2 is maximum feasible on the basis of these respective factors. NHTSA also considered several sensitivity cases by varying different inputs and concluded that, even when varying inputs resulted in changes to net benefits, those changes were not significant enough to alter the tentative conclusion that Alternative 2 is maximum feasible.

Finally, NHTSA has computed “annualized” benefits and costs relative to the No-Action Alternative, as follows:

For this and similar tables in this section, net benefits may differ from benefits minus costs due to rounding.

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Though NHTSA is prohibited from considering the availability of certain flexibilities in making its determination about the levels of CAFE standards that would be maximum feasible, manufacturers have a variety of flexibilities available to aid their compliance. NHTSA is proposing certain changes to these flexibilities and other features of the CAFE program as shown in Table I-6, and as described further in Section VI of this preamble.

BILLING CODE 4910-59-P

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BILLING CODE 4910-59-C

The

following sections of this preamble discuss the technical foundation for NHTSA's analysis, the regulatory alternatives considered in this proposed rule, the estimated effects of the regulatory alternatives, the basis for NHTSA's tentative conclusion that the proposed standards are maximum feasible, and NHTSA's approach to compliance and enforcement. The extensive record for this action consists of this proposed rule, a Draft Technical Support Document (Draft TSD), a Preliminary Regulatory Impact Analysis (PRIA), and a Draft SEIS, along with extensive analytical documentation, supporting references, and many other resources. Most of these resources are available on NHTSA's website,

and other references not available on NHTSA's website can be found in the rulemaking docket, the docket number of which is listed at the beginning of this preamble. NHTSA seeks comment on all aspects of this proposal and seeks comment on particular topics where indicated in each Section.

DOT will update the CAFE civil penalties regulations in 49 CFR 578.6(h) to reflect the statutory amendment in section 40006 of Public Law 119-21 in the next DOT-wide annual civil penalties update rulemaking.

See NHTSA, Corporate Average Fuel Economy, Last revised: 2023,

https://www.nhtsa.gov/laws-regulations/corporate-average-fuel-economy

(accessed: Sept. 10, 2025).

II. Technical Foundation for the NPRM Analysis

A. Why is NHTSA conducting this analysis?

When NHTSA promulgates new regulations or amends its existing regulations, it generally presents an analysis that estimates the impacts of those regulations, including the impacts of other regulatory alternatives it considered during the rulemaking. These analyses derive from statutes such as the APA

and the National Environmental Policy Act (NEPA),

from Executive orders (such as E.O. 12866),

and from other administrative guidance (

e.g.,

Office of Management and Budget (OMB) Circular A-4).

For this analysis in particular, EPCA contains several requirements governing the scope and nature of fuel economy standard setting.

Among these, some have been in place since EPCA was first signed into law in 1975, some were added in the Alternative Motor Fuels Act of 1988 (AMFA)

and in the Energy Policy Act of 1992,

and others were added in 2007 when Congress

passed the EISA.

Most recently, One Big Beautiful Bill Act (OB3) amended EPCA's civil penalty provisions.

Codified in 5 U.S.C. 551-559.

Codified in 42 U.S.C. 4321-4347.

Regulatory Planning and Review, 58 FR 51735 (Oct. 4, 1993).

Office of Management and Budget, Circular A-4 (Sept. 17, 2003), available at:

https://www.whitehouse.gov/wp-content/uploads/2025/08/CircularA-4.pdf

(accessed Sept. 10, 2025).

Public Law 94-163, 89 Stat. 871 (Dec. 22, 1975).

https://www.govinfo.gov/content/pkg/STATUTE-89/pdf/STATUTE-89-Pg871.pdf.

Public Law 100-494, 102 Stat. 2441 (Oct. 14, 1988).

https://www.govinfo.gov/content/pkg/STATUTE-102/pdf/STATUTE-102-Pg2441.pdf.

Public Law 102-486, 106 Stat. 2776 (Oct. 24, 1992).

https://www.govinfo.gov/content/pkg/STATUTE-106/pdf/STATUTE-106-Pg2776.pdf.

Public Law 110-140, 121 Stat. 1492 (Dec. 19, 2007).

https://www.govinfo.gov/content/pkg/STATUTE-121/pdf/STATUTE-121-Pg1492.pdf.

Public Law 119-21, 139 Stat. 72 (July 4, 2025).

https://www.congress.gov/119/plaws/publ21/PLAW-119publ21.pdf.

These statutes contain a variety of requirements for which NHTSA seeks to account in its analysis. NHTSA captures all of these requirements by presenting an analysis that spans a meaningful range of regulatory alternatives; that quantifies a range of technological, economic, and environmental impacts; and that does so in a manner that accounts for various express statutory requirements for the CAFE program (

e.g.,

passenger cars and light trucks must be regulated separately; and the standard for each fleet must be set at the maximum feasible level in each model year). NHTSA's standards are thus supported by, though not dictated by, extensive analysis of potential impacts of the regulatory alternatives under consideration. Together with this preamble, a Draft TSD, a PRIA, and a Draft SEIS provide a detailed enumeration of related analysis methods, estimates, assumptions, and results. These additional analyses can be found in the rulemaking docket for this proposed rule and on NHTSA's website.

Docket Nos. NHTSA-2025-0491; NHTSA-2025-0490.

See NHTSA, Corporate Average Fuel Economy, Last revised: 2023, available at:

https://www.nhtsa.gov/laws-regulations/corporate-average-fuel-economy

(accessed: Sept. 10, 2025).

This section provides further detail on the key features and components of NHTSA's standard-setting (also known as “constrained”) analysis. NHTSA's standard-setting analysis reflects statutory limitations on what NHTSA can consider when determining maximum feasible CAFE standards. In determining maximum feasible fuel economy levels, “the Secretary of Transportation—(1) may not consider the fuel economy of dedicated automobiles; (2) shall consider dual fueled automobiles to be operated only on gasoline or diesel fuel; and (3) may not consider, when prescribing a fuel economy standard, the trading, transferring, or availability of credits.”

NHTSA also conducts an “unconstrained” CAFE Model analysis to evaluate, as required by NEPA, the reasonably foreseeable environmental effects of its proposed action and a reasonable range of alternatives that meet the purpose and need for the proposed action.

The technical assumptions for EIS simulations are discussed in the Draft EIS Appendix C.

49 U.S.C. 32902(h).

42 U.S.C. 4332.

This section also describes how NHTSA's analysis has been constructed specifically to reflect other governing law applicable to CAFE standards, reviews how NHTSA's analysis has been updated to represent relevant statutory provisions more closely, and describes additional technical work recently conducted by the agency. The analysis for this proposed rule aids NHTSA in implementing its statutory obligations, including the weighing of various considerations, by informing decision-makers about the estimated effects of different regulatory alternatives.

1. What are the key components of NHTSA's analysis?

NHTSA's analysis makes use of a range of data (

i.e.,

observations of things that have occurred), estimates (

i.e.,

things that are unknown or may occur in the future), and models (

i.e.,

methods for making estimates). Two examples of

data

include (1) records of actual odometer readings used to estimate annual mileage accumulation at different vehicle ages and (2) CAFE compliance data used as the foundation for the “reference fleet” containing, among other things, production volumes and fuel economy levels of specific configurations of specific vehicle models produced for sale in the United States. Two examples of

estimates

include (1) forecasts of future gross domestic product (GDP) growth used, with other estimates, to forecast future vehicle sales volumes and (2) technology cost estimates, which include estimates of the technologies' “direct cost,” marked up by a “retail price equivalent” factor, to estimate the ultimate cost to consumers of a given fuel-saving technology, and an estimate of “cost learning effects” (

i.e.,

the tendency that it will cost a manufacturer less to apply a technology as the manufacturer gains more experience doing so).

In coordination with the DOT Volpe National Transportation Systems Center (Volpe or the Volpe Center), NHTSA uses the CAFE Compliance and Effects Modeling System (CAFE Model or the Model) to simulate and analyze manufacturers' potential responses to new CAFE standards and to estimate various impacts of those responses. NHTSA has used the CAFE Model to perform analyses supporting every CAFE rulemaking since 2001. Working together, NHTSA and Volpe ensure that the CAFE Model's operation reflects the statutory directives discussed in more detail in Section II below.

The CAFE Model first estimates how vehicle manufacturers might respond to a given regulatory scenario; from that potential compliance solution, the system estimates what impact that response will have on fuel consumption, emissions, safety impacts, and economic externalities. The following section summarizes information necessary to understand the analysis, while Draft TSD Chapter 2 and the CAFE Model Documentation present additional details on the Model's operation.

The CAFE Model may be characterized as an integrated system of models that estimate the impact of various policy options. For example, one model estimates manufacturers' responses, another estimates resultant changes in total vehicle sales, and still another estimates resultant changes in fleet turnover (

i.e.,

scrappage). Importantly, the modeling system does not determine the form or stringency of the standards, which must be developed in consideration of statutory factors that must be balanced by policy-makers. Instead, the CAFE Model applies inputs specifying the form and stringency of standards to be analyzed and produces outputs showing the impacts of manufacturers working to meet those standards, which become part of the basis for comparing different potential stringencies. A regulatory scenario, meanwhile, involves specification of the form, or shape, of the standards (

e.g.,

flat standards, or linear or logistic attribute-based standards), scope of passenger car and light truck regulatory classes, and stringency of the standards for each model year to be analyzed. For example, a regulatory scenario may define standards for a particular class of vehicles that increase in stringency by a given percent per year for a given number of consecutive years.

Manufacturer compliance simulation and the ensuing effects estimation, collectively referred to as compliance modeling, encompass numerous subsidiary elements. Compliance simulation begins with a detailed user-provided initial forecast of the vehicle models offered for sale during the simulation period.

The compliance simulation then attempts to bring each

manufacturer into compliance with the standards defined by the regulatory scenario contained within an input file developed by the user.

Because the CAFE Model is publicly available, anyone can develop their own initial forecast (or other inputs) for the Model to use. The DOT-developed Market Data Input File that contains the forecast for this proposed rule is available on NHTSA's website at

https://www.nhtsa.gov/corporate-average-fuel-economy/cafe-compliance-and-effects-modeling-system.

Estimating impacts involves calculating resulting changes in new vehicle costs, estimating a variety of costs (

e.g.,

for fuel expenditures or reduced or increased technology costs) and effects (

e.g.,

gallons of fuel used by the fleet) occurring as vehicles are driven over their lifetimes before eventually being scrapped, and estimating the monetary value of these effects. Estimating impacts also involves consideration of consumer responses (

e.g.,

the impact of vehicle fuel economy, operating costs, and vehicle price on consumer demand for light-duty vehicles). Both basic analytical elements involve the application of many inputs. Many of these inputs are developed outside of the Model and not by the Model. For example, the Model applies fuel price projections from DOE; it does not estimate fuel prices.

NHTSA also uses EPA's Motor Vehicle Emission Simulator (MOVES) model to estimate “vehicle” or “downstream” emission factors for criteria pollutants

and uses four DOE and DOE-sponsored models to develop inputs to the CAFE Model, including three developed and maintained by DOE's Argonne National Laboratory (Argonne). The agency uses the National Energy Modeling System (NEMS) of DOE's Energy Information Administration (EIA) to estimate fuel prices

and uses Argonne's Greenhouse gases, Regulated Emissions, and Energy use in Transportation (GREET) Model to estimate emissions rates from fuel production and distribution processes.

DOT also sponsors Argonne to run its Autonomie full-vehicle modeling and simulation system to estimate the fuel economy impacts for over a million combinations of technologies and vehicle types.

The Draft TSD and PRIA describe details of the agency's use of these models. In addition, as discussed in the Draft SEIS accompanying this proposed rule, NHTSA relied on a range of models to estimate various environmental impacts.

See

https://www.epa.gov/moves.

This proposed rule uses version MOVES5 (the latest version at the time of analysis), available at

https://www.epa.gov/moves/latest-version-motor-vehicle-emission-simulator-moves.

See

https://www.eia.gov/outlooks/aeo/.

This proposed rule uses fuel prices estimated using the Annual Energy Outlook (AEO) 2025 version of NEMS. See

https://www.eia.gov/outlooks/aeo/tables_ref.php.

Information regarding GREET is available at

https://greet.anl.gov/.

This proposed rule uses the R&D GREET 2023 version.

As part of the Argonne simulation effort, individual technology combinations simulated in Autonomie were paired with Argonne's BatPaC model to estimate the battery cost associated with each technology combination based on characteristics of the simulated vehicle and its level of electrification. Information regarding Argonne's BatPaC model is available at

https://www.anl.gov/cse/electrochemical-chemical-TEA.

In addition, the impact of engine technologies on fuel consumption, torque, and other metrics was characterized using GT-POWER simulation modeling in combination with other engine modeling that was conducted by IAV Automotive Engineering, Inc. (IAV). The engine characterization “maps” resulting from this analysis were used as inputs for the Autonomie full-vehicle simulation modeling. Information regarding GT-POWER is available at

https://www.gtisoft.com/gt-power/.

To prepare for the analysis that supports this proposed rule, DOT has continued to refine and expand the capabilities of the CAFE Model. As examples, and as discussed in more detail below, the reference fleet uses mid-MY 2024 compliance data (the most recent available data at the time of the analysis) and includes the capability (in addition to capabilities integrated into the modeling system) to account for proposed changes to the regulatory vehicle classification definitions. The analysis also employs separate input files for the modeling runs that NHTSA uses for its standard-setting analysis, which excludes the 49 U.S.C. 32902(h) factors that NHTSA cannot consider (constrained analysis), and the modeling runs that NHTSA uses for its analysis of impacts under the National Environmental Policy Act, which does not exclude the 49 U.S.C. 32902(h) factors (unconstrained analysis), and those input files have been updated accordingly. Common to both analyses are routine updates to dollar year values (

e.g.,

2021$ to 2024$) or routine updates to gas price projections. Some other updates, like updates to manufacturer credit banks, are confined to the unconstrained analysis only and are discussed further in the Draft SEIS Appendix C. The values of many inputs remain uncertain, and NHTSA has conducted sensitivity analyses around selected inputs to attempt to capture some of that uncertainty. These changes reflect DOT's long-standing commitment to ongoing refinement of its approach to estimating the potential impacts of new CAFE standards. These and other updated analytical inputs are outlined in Section II below and discussed in detail in the Draft TSD and PRIA.

2. How do statutory requirements shape NHTSA's analysis?

Multiple requirements govern the scope and nature of CAFE standard setting; the specific requirements regarding the technical characteristics of CAFE standards and the analysis thereof include, but are not limited to, the following:

Corporate Average Standards:

49 U.S.C. 32902 requires that standards apply to the average fuel economy levels achieved by each manufacturer's fleet of vehicles produced for sale in the United States. The CAFE Model calculates the average fuel economy of each manufacturer's fleet based on estimated production volumes and characteristics, including fuel economy levels of distinct vehicle models that could be produced for sale in the United States.

Separate Standards for Passenger and Non-Passenger Automobiles:

49 U.S.C. 32902 requires DOT to set CAFE standards separately for passenger and non-passenger automobiles. The CAFE Model accounts separately for passenger and non-passenger automobiles, including differentiated standards and compliance.

Attribute-Based Standards:

49 U.S.C. 32902 requires DOT to define CAFE standards for passenger and non-passenger automobiles as mathematical functions expressed in terms of one or more attributes related to fuel economy. This means that, for a given manufacturer's fleet of vehicles produced for sale in the United States in a given regulatory class and model year, the applicable minimum CAFE requirement (

i.e.,

the numerical value of the requirement) is computed based on the applicable mathematical function as well as the mix and attributes of vehicles in the manufacturer's fleet. The CAFE Model accounts for such functions and vehicle attributes explicitly.

Separately Defined Standards for Each Model Year:

49 U.S.C. 32902 requires DOT to set CAFE standards (separately for passenger and non-passenger automobiles) at the maximum feasible levels in each model year. The CAFE Model represents each model year explicitly and accounts for the production relationships between model years. For example, a new engine first applied to a given vehicle model/configuration in MY 2030 most likely will be retained in MY 2031 for that same vehicle model to reflect the fact that manufacturers do not apply brand-new engines to a given vehicle model every single year. The CAFE Model is designed to account for this reality, while still respecting applicable statutory constraints.

Separate Compliance for Domestic and Imported Passenger Car Fleets:

49 U.S.C. 32904 requires EPA to determine average fuel economy separately for each manufacturer's fleet of domestic passenger cars and imported passenger

cars. A passenger car is considered to be domestic or imported based on the definitions provided in 49 U.S.C. 32904. The CAFE Model accounts explicitly for this requirement when simulating manufacturers' potential responses to CAFE standards.

Minimum CAFE Standards for Domestic Passenger Car Fleets:

49 U.S.C. 32902 requires that domestic passenger car fleets also meet a minimum CAFE standard, which is calculated as 92 percent of the average fuel economy projected by the Secretary for the combined passenger car fleet manufactured for sale in the United States by all manufacturers in the model year. This projection is published at the time the standard is promulgated. The CAFE Model accounts explicitly for this requirement.

Statutory Basis for Stringency:

49 U.S.C. 32902 requires DOT to set CAFE standards for passenger and non-passenger automobiles at the maximum feasible levels, considering technological feasibility, economic practicability, the need of the U.S. to conserve energy, and the impact of other motor vehicle standards of the Government on fuel economy. The analysis and balancing of these factors necessarily changes in light of current and projected economic and market conditions. Accordingly, NHTSA has continued to expand and refine its qualitative and quantitative analysis to account for these statutory factors in light of such conditions. For example, the simulations of technology effectiveness reflect the agency's judgment that it would not be economically practicable, appropriate, or cost effective for a manufacturer to “split” an engine shared among many vehicle models/configurations into myriad versions each optimized to a single vehicle model/configuration.

Civil Penalties for Noncompliance:

49 U.S.C. 32912 (and implementing regulations) prescribe a rate (in dollars per tenth of a mile per gallon (mpg)) at which the Secretary is to levy civil penalties if a manufacturer fails to comply with a CAFE standard for a given fleet in a given model year. When civil penalties are applicable (

i.e.,

when they are not set by statute to a value of $0, as they have been at the time of this analysis of the proposed rule), the CAFE Model will calculate civil penalties for CAFE shortfalls (if directed to do so by the user). However, as stated, civil penalty values are currently set by statute to a value of $0; therefore, the CAFE Model's calculations will always result in zero civil penalties.

Dual-Fueled and Dedicated Alternative Fuel Vehicles:

For purposes of calculating CAFE standards used to determine passenger and non-passenger automobile fleet compliance, 49 U.S.C. 32905 and 32906 specify methods for calculating the fuel economy levels of vehicles operating on alternatives to gasoline or diesel fuels. The CAFE Model can account for these requirements explicitly for each relevant vehicle model. However, 49 U.S.C. 32902 also prohibits consideration of the fuel economy of dedicated alternative fuel vehicle (AFV) models (or the non-gasoline or non-diesel calculated fuel economy of dual-fueled AFVs) when NHTSA determines what levels of passenger and non-passenger automobile CAFE standards are maximum feasible. Therefore, the CAFE Model is run in a manner that excludes dedicated AFV technologies and limits the consideration of a dual-fueled AFV's fuel economy to only its gasoline or diesel operation. NHTSA operates the Model with this limitation when performing the analysis that is used to inform the setting of standards. The CAFE Model can also be run without this analytical constraint, and the agency does so in the NEPA analysis described below.

Creation and Use of Compliance Credits:

49 U.S.C. 32903 provides that manufacturers may earn CAFE “credits” by achieving an average fuel economy level beyond that required of a given fleet in a given model year and specifies how these credits may be used to offset the amount by which a different fleet falls short of its corresponding requirement. These provisions allow credits to be “carried forward” a maximum of five model years, “carried back” a minimum of three model years, transferred between regulated classes, and traded between manufacturers. However, credit use is also subject to specific limits: the statute caps the amount of credit that can be transferred between a manufacturer's fleets and prohibits manufacturers from applying traded or transferred credits to offset a failure to achieve the minimum standard for domestic passenger automobiles. The CAFE Model has the capability to simulate manufacturers' potential use of credits carried forward from prior model years or transferred from other fleets;

however, this capability is not used in the standard-setting analysis because 49 U.S.C. 32902 prohibits consideration of manufacturers' potential application of CAFE compliance credits when setting maximum feasible CAFE standards for passenger and non-passenger automobiles.

Note that the CAFE Model does not simulate the potential for manufacturers to carry CAFE credits back (

i.e.,

borrow) from future model years or acquire and use CAFE compliance credits from other manufacturers. NHTSA believes that there is significant uncertainty in how manufacturers may choose to use these particular flexibilities in the future: for example, while it is reasonably foreseeable that a manufacturer who over-complies in 1 year may “coast” through several subsequent years relying on that prior improvement rather than continuing to make technology improvements year after year, it is harder to assume with confidence that manufacturers will rely on future technology investments to offset prior-year shortfalls, or whether and how manufacturers will trade credits with market competitors rather than make their own technology investments.

National Environmental Policy Act (NEPA):

The Draft SEIS accompanying this proposed rule documents changes in fuel use and emissions as estimated using the CAFE Model and also documents corresponding estimates—based on the application of other models documented in the Draft SEIS—of environmental impacts of the regulatory alternatives under consideration.

3. What updated capabilities and assumptions does the current Model reflect as compared to the version used in the analysis of the 2024 final rule?

DOT has continued its ongoing effort to refine and expand the capabilities of the CAFE Model for use in analyzing regulatory alternatives as considered in this proposal. Any analysis of regulatory actions that will be implemented several years in the future, and whose benefits and costs accrue over decades, requires many assumptions. Over such time horizons, many, perhaps even most, of the relevant assumptions in such an analysis are inevitably uncertain. To help address this, NHTSA updates the assumptions used in each successive CAFE analysis to reflect the current state of the world more accurately and to apply the best current estimates of future conditions. Accordingly, since the 2024 final rule, DOT has made the following changes to the CAFE Model and its inputs:

• Updating the Market Data Input File to reflect the change in analysis fleet from MYs 2022-2024;

• Updating algorithms and settings to remove statutorily prohibited inputs from the standard-setting analysis and to select between different types of analyses (

i.e.,

constrained and unconstrained);

• Updating the base dollar year from 2021$ to 2024$;

• Updating the capability to exclude plug-in hybrid electric vehicle (PHEV) electricity usage when PHEV fuel economy operation is in gasoline-only mode for standard setting;

• Updating the modeling capability to allow for vehicle reclassification;

• Updating the Market Data Input File to include vehicle reclassification;

• Updating the Model to use a bracketed costing approach to determine prices for the five levels of mass reduction (MR);

• Updating the Scenarios Input File to remove AC and OC FCIVs;

• Updating the Market Data Input File to include advanced truck credits for MY 2024 vehicles, noting that those credits sunset after MY 2024 and are therefore only applicable to that one year;

• Updating the Parameters Input File to set the social cost of carbon at zero;

• Updating the Parameters Input File for changes in other economic variables;

• Updating the Scenarios Input File with an adjusted tax credit phase-out timeframe;

• Updating the Scenarios Input File to set civil penalties to zero;

• Updating selected economic assumptions:

○ Rebound elasticity;

○ Payback period;

○ Value of travel time per vehicle; and

○ Numerous other updates based on the 2025 AEO; and

• Updating emission rates based on EPA's “MOVES5” model.

These and other updated analytical inputs are discussed in the remainder of this section and in detail in the Draft TSD.

B. What is NHTSA analyzing?

NHTSA is analyzing the effects of different potential CAFE standards on industry, consumers, and society at large. These different potential standards are described as “regulatory alternatives,” and, amongst the regulatory alternatives, NHTSA identifies which ones the agency is proposing to select. EPCA, as amended by EISA, expressly requires that CAFE standards for passenger cars and light trucks be based on one or more vehicle attributes related to fuel economy and be expressed in the form of a mathematical function.

Thus, the standards (and the regulatory alternatives) for passenger cars and light trucks take the form of fuel economy targets expressed as functions of vehicle footprint (the product of vehicle wheelbase and average track width) that are separate for passenger cars and light trucks.

49 U.S.C. 32902(a)(3)(A).

Under the footprint-based standards, the function defines a fuel economy performance target for each unique footprint combination within a car or truck model type. Using the functions, each manufacturer thus will have an average fuel economy standard for each year that is unique to each of its regulatory fleets (

i.e.,

passenger automobiles and non-passenger automobiles, consistent with 49 U.S.C. 32902(b)), based on the footprint and production volumes of the vehicle models produced by that manufacturer. The functions are negatively sloped, so that larger vehicles (

i.e.,

vehicles with larger footprints) will generally be subject to lower mpg targets than smaller vehicles. This is because smaller vehicles are typically more capable of achieving higher levels of fuel economy, because they tend not to require as much energy to propel the mass necessary to perform their driving task. Although a manufacturer's fleet average standard could be estimated throughout the model year based on the projected production volume of its vehicle fleet (and is estimated as part of EPA's certification process), the standards with which the manufacturer must comply are determined by its final model year production figures. A manufacturer's calculation of its fleet average standards, as well as its fleets' average performance at the end of the model year, will thus be based on the production-weighted average target and performance of each model in its fleet.

As discussed in prior rulemakings, a manufacturer may have some vehicle models that exceed their target and some that are below their target. Compliance with a fleet average standard is determined by comparing the fleet average standard (based on the production-weighted average of the target levels for each model) with fleet average performance (based on the production-weighted average of the performance of each model). This is inherent in the statutory structure of CAFE, which requires NHTSA to set

corporate average

standards.

For passenger cars, consistent with prior rulemakings, NHTSA is defining fuel economy targets as shown in Equation II-1.

Equation II-1: Passenger Car Fuel Economy Footprint Target Curve

EP05DE25.016

Where:

TARGET

is the fuel economy target (in mpg) applicable to a specific vehicle model type with a unique footprint combination,

is a minimum fuel economy target (in mpg),

is a maximum fuel economy target (in mpg),

is the slope (in gallons per mile (or gpm) per square foot) of a line relating fuel consumption (the inverse of fuel economy) to footprint, and

is an intercept (in gpm) of the same line.

Here,

MIN

and

MAX

are functions that take the minimum and maximum values, respectively, of the set of included values. For example,

MIN

[40, 35] = 35 and

MAX

(40, 25) = 40, such that

MIN

[

MAX

(40, 25), 35] = 35.

For light trucks, also consistent with prior rulemakings, NHTSA is defining fuel economy targets as shown in Equation II-2.

Equation II-2: Light Truck Fuel Economy Footprint Target Curve

EP05DE25.017

Where:

TARGET

is the fuel economy target (in mpg) applicable to a specific vehicle model type with a unique footprint combination, and

a, b,

and

are as for passenger cars, but take values specific to light trucks.

Though the general model of the target function equation is the same for passenger cars and light trucks, and the

same for each model year, the parameters of the function equation differ for cars and trucks.

The parameters defining the general curve shapes have remained the same since the 2012 final rule. NHTSA periodically reconsiders whether to update the mathematical functions but in each prior instance concluded that the existing curves continued to represent the relationship between footprint and fuel economy reasonably. Consistent with the agency's past practice of reviewing the mathematical functions prior to each rulemaking, NHTSA re-examined the curve shapes for this proposal.

More specifically, NHTSA performed descriptive statistical analyses using manufacturer-reported data for the MY 2022 and MY 2024 fleets. NHTSA used the MY 2022 fleet for analysis of curve shapes relevant to the MYs 2022-2027 standards and used the MY 2024 “reclassified” fleet for analysis of curve shapes relevant to the MYs 2028-2031 standards. As discussed in more detail in Draft TSD Chapter 1, the proposed updates to NHTSA's vehicle classification regulations beginning in MY 2028 have material impacts on the relationship between fuel economy and footprint for each regulatory class, as expressed by the standards-defining functions.

To estimate the relationship between fuel economy and footprint and to maintain general consistency with analyses of past rules (and the conformance to statutory prohibitions), the agency excluded all diesel engine vehicles and all plug-in electric vehicles, which include plug-in hybrid electric vehicles, battery electric vehicles (BEV), and fuel cell electric vehicles (FCEV), and applied weighting and other adjustments to the fuel consumption and footprint data. Table II-1 summarizes the methodological approaches that NHTSA considered for reassessing the footprint curves.

EP05DE25.018

The maximum technology fleet was simulated with the CAFE Model, assuming a MY 2024 fleet and maximum allowable technology application.

NHTSA believes that the ordinary least-squares (OLS) regression framework continues to be an appropriate method for estimating the relationship of footprint to fuel economy. While the agency relied on the minimum absolute deviation (MAD) regression framework in the 2010 final rule to address the effects of “outlier” vehicles in the fleet, the agency addresses outlier vehicles in this reconsideration through technology-based exclusions (

i.e.,

by excluding diesels, PHEVs, BEVs, and FCEVs, as mentioned above) and data normalization through the application of controls, including curb weight (CW) to footprint, horsepower (HP) to CW, and both together, depending on the regulatory fleet under consideration, as it has in each of its CAFE rulemaking actions since 2012. The curves also reflect updated fleet data to reset the “cutpoints,” or the places at the lowermost and uppermost bounds of vehicle footprint distributions where the standards remain flat (

i.e.,

the mpg target does not continue to increase as footprint decreases, and vice versa). The resulting footprint curves are shown in Section III's discussion of the regulatory alternatives.

As discussed in Draft TSD Chapter 1, NHTSA considers a variety of technical and policy issues when determining the footprint curve shape in any CAFE rulemaking action. For example, standards that decrease sharply with increasing footprint could create incentives for manufacturers to upsize vehicles, since small changes in vehicle footprint would result in a significant change in the vehicle's fuel economy target; conversely, flatter standards could create a significant amount of additional technology burden for larger vehicles to meet fuel economy targets like those of smaller vehicles. That said, NHTSA performed an analysis for the 2024 final rule showing that vehicle footprints, within vehicle types, have been stable on a sales-weighted basis since MY 2012.

The biggest increase to within-type footprints was for the sedan/wagon category, which increased by 3.4 percent (or about 2 square feet) from 2012 (for reference, a 1.5-square foot increase would equate to about a 2-inch increase in the track width of a MY 2022 Toyota Corolla). NHTSA concluded that the disconnect between vehicle class-level characteristics and what was being perceived at the fleet level (

i.e.,

vehicles seemingly getting larger) was traceable to the increase in the share of fleet vehicles classified as light trucks relative to the share of passenger cars. Available data indicate that the use of footprint as an attribute did not appear to lead to manufacturers significantly altering the size of their vehicles within vehicle classes and that the major shift in fleet share was not a result of the shape of the footprint curves.

NHTSA, Technical Support Document: Corporate Average Fuel Economy Standards for Passenger Cars and Light Trucks for Model Years 2027 and Beyond and Fuel Efficiency Standards for Heavy-Duty Pickup Trucks and Vans for Model Years 2030 and Beyond, NHTSA: Washington, DC, pp. 1-20 (2024).

The footprint curve updates for this proposal are intended to ensure that the agency appropriately captures the footprint-to-fuel economy relationship using the most current data. As discussed in Draft TSD Chapter 1, the observed relationship between footprint and fuel economy for both the passenger car and light truck fleets is on average “flatter” (

i.e.,

on average, the fuel economy did not vary as much across footprint levels) than the MY 2008 fleet used to create the footprint curves for the past several rules. While the technical concerns and policy trade-offs associated with the curve shapes still hold to some extent, NHTSA believes it is more likely, as shown from the agency's 2024 analysis and the updated analysis presented in Section VI, that any shift in vehicle attributes present in the market over time has not been due to the shapes of curves or the use of footprint as the relevant attribute. NHTSA seeks comments on this belief, as well as the updated footprint curve shape analysis, discussed in more detail in Draft TSD Chapter 1.

Finally, the required CAFE level applicable to a passenger car (either domestic or import) or light truck fleet in a given model year is determined by calculating the production-weighted harmonic average of fuel economy targets applicable to specific vehicle model configurations in the fleet, as shown in Equation II-3.

Equation II-3: Calculation for Required CAFE Level

EP05DE25.019

Where:

CAFE

required

is the CAFE level the fleet is required to achieve,

refers to specific vehicle model configurations in the fleet,

PRODUCTION

is the number of model configuration

produced for sale in the United States, and

TARGET

FE, i

is the fuel economy target (as defined above) for model configuration

Additional details about the specific values defining the mathematical functions and visual representations of the fuel economy target curves are presented in Section III, below.

C. What inputs does the compliance analysis require?

The first step in the agency's analysis of the effects of different levels of fuel economy standards is the compliance simulation. As used throughout this rulemaking, “compliance simulation” means the simulation of how manufacturers could comply with different levels of CAFE standards by adding fuel economy-improving technology to an existing fleet of vehicles, using the CAFE Model. The CAFE Model uses a variety of data, including data provided by manufacturers, to simulate final fleet sales and performance.

When NHTSA uses the phase “the Model” throughout this section, NHTSA is referring to the CAFE Model. Any other model is specifically named.

At the most basic level, a model is a set of equations, algorithms,

or other calculations used to make predictions about a complex system. A model may consider various inputs, such as technology costs or other relevant factors, and use those inputs to generate output predictions. NHTSA used two separate approaches for which it is proposing to amend the existing CAFE standards, one for MYs 2022-2026 and one for MYs 2027-2031. The sections

below discuss the inputs each of those analyses used.

See Merriam-Webster “algorithm.” Broadly, an algorithm is a step-by-step procedure for solving a problem or accomplishing some end. More specifically, an algorithm is a procedure for solving a mathematical problem (as of finding the greatest common divisor) in a finite number of steps that frequently involves repetition of an operation.

1. What inputs does the analysis require for 2022-2026?

For the MYs 2022-2026 analysis, NHTSA has performed two exercises: first, it has re-evaluated the statistical model used to determine the shape (

i.e.,

slope, intercept, and cutpoints) of the target functions for passenger cars and light trucks. Based on its preferred choice of shape, NHTSA has evaluated the compliance position of manufacturers in MYs 2022-2024 under alternative stringencies and compared results to the manufacturers' achieved average fuel economy in these years. For both exercises, NHTSA relies on compliance data from manufacturer mid-year compliance reports. For its curve fitting analysis, NHTSA uses vehicle model level data on vehicle attributes, including footprint, HP, CW, and 2-cycle fuel economy. NHTSA also uses mid-year estimates of model sales from manufacturer compliance data for this exercise. NHTSA's curve fitting analysis is described in greater detail in Draft TSD Chapter 1. For NHTSA's comparison of achieved fuel economy and proposed standards levels, the agency uses compliance data at the model level for vehicle footprint, 2-cycle fuel economy, and mid-year estimates of vehicle sales.

For MYs 2022-2024, NHTSA uses each proposed standard to calculate vehicle model target function values for each vehicle model in the standard-setting fleet.

Consistent with past rulemakings, the agency uses piecewise linear functions of vehicle footprint, which map to a target value of fuel consumption rate in gallons-per-mile.

NHTSA determines a vehicle's target fuel economy level in miles per gallon for a given set of standards, and then takes the reciprocal of this value. NHTSA determines the CAFE standards for each manufacturer at the regulatory class level under each alternative by taking the sales-weighted harmonic mean of the relevant models produced by the manufacturer in each regulatory class in each model year. The agency repeats these calculations for each model year under consideration to determine a single value for each regulatory class in which the manufacturer produced vehicles.

Per 49 U.S.C. 32902(h), dedicated alternative fueled vehicles, such as EVs, are excluded from this analysis. For duel-fueled vehicles, the analysis uses a fuel economy value for the vehicles operating only on gasoline or diesel fuel.

Id.

See Chapter 1.2 of the Draft TSD discussing footprint functions.

NHTSA also computes the MDPCS for each model year by taking the sales-weighted harmonic mean of the model-level target function values for all vehicles in the passenger car fleet in that model year and multiplying the value by 92 percent.

49 U.S.C. 32902(b)(4).

NHTSA determines each manufacturer's achieved fuel economy in miles per gallon separately for each regulatory class using the sales-weighted average of the 2-cycle fuel economy values of all models produced by the manufacturer in the relevant regulatory class. NHTSA then compares this achieved value to the corresponding manufacturer regulatory class standard in each model year to determine whether the fleet of vehicles to which it corresponds would comply with each proposed standard in that model year. To determine the total number of vehicles out of compliance, NHTSA determines compliance for each manufacturer's regulatory fleet in each model year under each proposed alternative, and if a fleet is determined to be out of compliance, the agency sums the total number of vehicles sold in the non-compliant fleet.

As discussed in more detail in Section IV, NHTSA analyzes the difference between each manufacturer's fleet CAFE compliance value and the proposed standard. NHTSA has considered using the CAFE Model to simulate behavior for the MYs 2022-2026 compliance period to estimate how manufacturers and consumers could have responded to different CAFE standards. However, for MYs 2022-2025, production is already closed or is in process, and MY 2026 production plans likely are solidified and underway by the time of this NPRM's publishing. This type of analysis overestimates the ability of manufacturers to optimize in response to the proposed standards for these years and likely leads to different results from the actual outcomes. Thus, simulating a response and any monetized costs or benefits deriving from that do not represent real economic effects from the proposed change in policy.

2. What inputs does the compliance analysis require for 2027-2031?

For the MYs 2027-2031 amendment analysis, NHTSA used the CAFE Model to simulate manufacturers' potential responses to new CAFE standards and to estimate the various impacts of those responses on manufacturers and society. The Model considers various inputs, such as technology effectiveness data, technology costs, and other relevant factors, and uses those inputs to generate output predictions.

NHTSA attempts to ensure that the technology inputs and assumptions that go into the CAFE Model are based on sound science and reliable data and that NHTSA's reasons for using those inputs and assumptions are transparent and understandable to stakeholders. This section and the following section discuss at a high level how the agency generates the technology inputs and assumptions that the CAFE Model uses for the compliance simulation.

The Draft TSD, CAFE Model Documentation, CAFE Analysis Autonomie Documentation,

and other technical reports supporting this proposed rule discuss the agency's technology inputs and assumptions in more detail.

As explained throughout this section, a NHTSA input is a specific number or datapoint used by the Model, and NHTSA's assumptions are based on judgment after careful consideration of available evidence. An assumption can be an underlying reason for the

use

of a specific datapoint, function, or modeling process. For example, an input might be the fuel economy value of the Ford Mustang, whereas the assumption is that the Ford Mustang's fuel economy value reported in Ford's CAFE compliance data should be used in NHTSA's modeling.

The Argonne report is titled “Vehicle Simulation Process to Support the Analysis for MY 2027 and Beyond CAFE and MY 2030 and Beyond HDPUV FE Standards.” However, for ease of use and consistency with the Draft TSD it is referred to as “CAFE Analysis Autonomie Documentation.”

NHTSA incorporates technology inputs and assumptions either directly in the CAFE Model or in the CAFE Model's various input files. The compliance simulation algorithm is at the heart of the CAFE Model's decisions about how to apply technologies to a manufacturer's vehicles to project how the manufacturer could meet CAFE standards. The compliance simulation algorithm consists of several equations that direct the Model to apply fuel economy-improving technologies to vehicles in a way that simulates how manufacturers might apply those technologies to their vehicles in the real world. The compliance simulation algorithm projects a cost-effective pathway for manufacturers to comply with different levels of CAFE standards, considering the technology present on manufacturers' vehicles now and what technology could be applied to their vehicles in the future. Embedded in the CAFE Model is the universe of technology options that the Model can consider and rules about the order in which it can consider those options, as well as estimates of how effective fuel economy-improving technology is on different types of vehicles (

e.g.,

sedan or pickup truck).

Technology inputs and assumptions are also located in all four of the CAFE Model Input Files. The Market Data Input File is a spreadsheet file that characterizes the fleet of vehicles used as the starting point for the CAFE Model. There is one row describing each vehicle model and model configuration manufactured for the United States market in a model year (or years) and input and assumption data that links those vehicles to technology and economic, environmental, and safety inputs and assumptions. The Technologies Input File identifies 71 technologies the agency uses in the analysis, along with information used to inform the compliance simulation and effects estimates, including phase-in caps to identify when and how widely each technology can be applied to specific types of vehicles, most of the technology costs (hybrid vehicle battery costs are provided in a separate file), and the fuel share percentage for PHEV to capture the charge sustaining operation. The Scenarios Input File provides the coefficient values defining the standards for each regulatory alternative

and other relevant information applicable to modeling each regulatory scenario.

Finally, the Parameters Input File contains mainly economic and environmental data.

The coefficient values are defined in PRIA Chapter 3 for the CAFE standard.

This file also includes information about the amount of fuel consumption improvement values a manufacturer may generate for compliance purposes for model years in which a manufacturer may generate them.

See CAFE Model Documentation for a detailed discussion of what inputs are held in each of the input data files.

NHTSA generates these technology inputs and assumptions in several ways, including using data submitted by vehicle manufacturers pursuant to their CAFE reporting obligations; public data on vehicle models from manufacturer websites, press materials, marketing brochures, and other publicly available information; collaborative research, testing, and modeling with other Federal agencies, like Argonne; and research, testing, and modeling with independent organizations, like IAV GmbH Ingenieurgesellschaft Auto und Verkehr (IAV), Southwest Research Institute (SwRI), National Academy of Sciences (NAS), and FEV North America. NHTSA also considers the work done to develop inputs and assumptions for prior rules to the extent it is still relevant and applicable; feedback from stakeholders on prior rules and from meetings conducted before the commencement of this proposed rule; and NHTSA's own engineering judgment. NHTSA uses the term “engineering judgment” throughout this rulemaking to refer to decisions made by a team of NHTSA engineers and analysts. This judgment is based on their experience working in the automotive industry and other relevant fields and assessment of all the data sources described above. Most importantly, the agency uses engineering judgment to assess how best to represent vehicle manufacturers' potential responses to different levels of CAFE standards within the boundaries of the agency's modeling tools, as “a model is meant to simplify reality in order to make it tractable.”

In other words, NHTSA uses engineering judgment to concentrate potential technology inputs and assumptions from millions of discrete data points from hundreds of sources into four external input files and three datasets integrated into the CAFE Model. How the CAFE Model decides to apply technology (

i.e.,

the compliance simulation algorithm), has been developed using engineering judgment, considering factors that manufacturers consider when they add technology to vehicles in the real world. The specific technology inputs and assumptions are discussed in more detail in the following sections and in the associated technical documentation.

Chem. Mfrs. Ass'n v. EPA, 28 F.3d 1259, 1264-65 (D.C. Cir. 1994) (citing Milton Friedman, in Friedman, M., The Methodology of Positive Economics, in Essays in Positive Economics 3, University of Chicago Press: Chicago, IL, pp. 14-15 (1953), available at:

https://www.wiwiss.fu-berlin.de/fachbereich/bwl/pruefungs-steuerlehre/loeffler/Lehre/bachelor/investition/Friedman_the_methology_of_positive_economics.pdf

(accessed: Sept. 10, 2025)).

a. Technology Options and Pathways

NHTSA begins the compliance analysis by defining the range of fuel economy-improving technologies that the CAFE Model could add to a manufacturer's vehicles in the U.S. market.

These are technologies that the agency believes are representative of what vehicle manufacturers currently use on their vehicles, and that vehicle manufacturers could use on their vehicles in the timeframe for the proposed standards (MYs 2027-2031). The technology options include engines, transmissions, hybridization, and road load technologies, which include MR, aerodynamic improvement (aerodynamic drag technology (AERO)), and tire rolling resistance (ROLL) reduction technologies.

40 CFR 86.1806-17, Onboard diagnostics; 40 CFR 86.1818-12, Greenhouse gas emission standards for light-duty vehicles, light-duty trucks, and medium-duty passenger vehicles; Commission Directive 2001/116/EC—European Union emission regulations for new LDVs—including passenger cars and light commercial vehicles (LCV).

Draft TSD Chapter 3 contains discussion on the technology tree and technologies available.

Adding a technology to the range of options that the CAFE Model can consider requires several data elements, including a broadly applicable technology definition, estimates of how effective that technology is at improving fuel economy on different vehicle types (

e.g.,

sedan or pickup truck), and the cost to apply that technology to each. Each technology the agency selects is designed to be representative of a wide range of specific technology applications used in the automotive industry. Some manufacturers' systems may perform better or worse than NHTSA's modeled systems, and some may cost more or less than NHTSA's modeled systems. However, selecting representative technology definitions for the agency's analysis ensures the agency captures a reasonable level of costs and benefits that would result from any manufacturer applying the technology.

NHTSA has been refining the technology options it considers since first developing the CAFE Model in 2002. In this context, “refining” means both adding and removing technology options depending on current technology availability and projected future availability in the U.S. market, while balancing a reasonable amount of modeling and analytical complexity. In recent years, the agency has refined the internal combustion engine (ICE) technology options, particularly the TURBO and HCR pathways, to reflect better the diversity of engines in the current fleet. Consistent with NHTSA's interpretation of EPCA/EISA, discussed further in Section II.0 and V, the agency includes several hybrid technologies to represent appropriately the diversity of current and anticipated future technology options while ensuring NHTSA's analysis remains consistent with statutory limitations prohibiting the consideration of EVs in establishing standards and considering only the gas or diesel operation of dual fueled automobiles.

The technology options do not include technologies NHTSA has determined will not be available in the rulemaking timeframe. As with past analyses, the agency does not include technologies unlikely to be feasible in the rulemaking timeframe, engine technologies designed for markets other than the United States market required to use unique gasoline,

or technologies

for which appropriate data are not available for the range of vehicles that the agency models in the analysis (

i.e.,

technologies that are still in the research and development phase and not ready for mass-market production). Each technology section below and Chapter 3 of the Draft TSD discuss these modeling decisions in detail.

In general, most vehicles produced for sale in the United States have been designed to use “regular” gasoline, or 87 octane. See EIA, Gasoline

Explained: What is octane?, Last revised: Nov. 17, 2022, available at:

https://www.eia.gov/energyexplained/gasoline/octane-in-depth.php

(accessed: Sept. 10, 2025).

In this analysis, the CAFE Model does not dictate or predict the technologies manufacturers must use to comply; rather, the CAFE Model outlines a technology pathway that manufacturers could use to meet the standards cost effectively. While NHTSA estimates the costs and benefits for different levels of CAFE standards based on a simulation of the technology manufacturers could apply in the rulemaking timeframe, it is entirely possible and reasonable that manufacturers may use different technology options to meet the agency's standards in the real world and may even use technologies that NHTSA does not include in the analysis. This is because NHTSA's standards do not mandate the application of any particular technology. Rather, NHTSA's standards are performance-based: manufacturers in the real world can and do use a range of compliance solutions that include technology application and encouraging sales shifts from one vehicle model or trim level to another.

The agency has determined that the 71 technology options included in the analysis strike a reasonable balance between representing the diversity of technology used by the entire industry and simplifying reality to make modeling workable.

Manufacturers could increase their production of one type of vehicle with higher fuel economy, like the hybrid version of a conventional vehicle model, to meet the standards. For example, Ford has conventional and hybrid versions of its F-150 pickup truck, and Toyota has conventional, hybrid, and plug-in hybrid versions of its RAV4 sport utility vehicle.

For each technology option, the analysis includes distinct technology cost and effectiveness values for 10 different types of vehicles, resulting in nearly half a million different technology effectiveness and cost data points.

Chapter 3 of the Draft TSD and Section II.0 below describe the technologies that NHTSA uses for the analysis. Each technology has a name that loosely corresponds to its real-world technology equivalent. NHTSA abbreviates the name to a short signifier for the CAFE Model to read. The agency organizes those technologies into groups based on technology type: basic and advanced engines, transmissions, hybridization, and road load technologies, which include MR, aerodynamic improvement, and low rolling resistance tire technologies.

NHTSA then organizes the groups into pathways. The pathways instruct the CAFE Model how and in what order to apply technology. In other words, the pathways define mutually exclusive technologies (

i.e.,

those that cannot be applied at the same time) and define the direction in which vehicles can advance as the Model evaluates which technologies to apply. The respective technology chapters in the Draft TSD and Section 4 of the CAFE Model Documentation include a visual of each technology pathway. In general, the paths are tied to ease of implementation of additional technology and how closely related the technologies are.

As an example, NHTSA's “Turbo Engine Path” consists of five different engine technologies that employ different levels of turbocharging technology. A turbocharger is essentially a small turbine driven by exhaust gases produced by the engine. As these gases flow through the turbocharger, they spin the turbine, which in turn spins a compressor that pushes more air into an engine's cylinders. Having more air in the engine's cylinders allows the engine to burn more fuel, which then creates more power, without needing a physically larger engine. In the agency's analysis, an engine that is turbocharged “downsizes,” or becomes smaller. Choosing to turbocharge an engine allows a manufacturer to maintain similar levels of performance to a larger, non-turbocharged engine with a smaller engine that uses less fuel to do the same amount of work. Allowing basic engines to be downsized and turbocharged instead of just turbocharged keeps the vehicle's utility and performance constant so that NHTSA can measure the costs and benefits of different levels of fuel economy improvements, rather than the change in different vehicle attributes. This concept of performance neutrality is discussed further, below.

The Model only allows forward movement along the technology pathways, adding more advanced technology as the Model moves through the technology tree. This ensures that a vehicle that uses a more advanced technology cannot downgrade to a less advanced version of the technology or ensures that a vehicle does not switch to technology that is significantly technically different. This progressive order also realistically represents how manufacturers often start with the lowest and most cost-effective technologies and generally advance along particular technology pathways. As an example, if a vehicle in the compliance simulation begins with a TURBOD engine—a turbocharged engine with cylinder deactivation—it cannot adopt a TURBO0 engine.

Similarly, this vehicle with a TURBOD engine cannot adopt an advanced cylinder deactivation on a dual-overhead camshaft engine (ADEACD) engine.

As an example of NHTSA's rationale for ordering technologies on the technology tree, an engine could potentially be changed from TURBO0 to TURBO2 without redesigning the engine block or requiring significantly different expertise to design and implement. A change to ADEACD likely would require a different engine block that might not fit in the engine bay of the vehicle without a complete redesign and different technical expertise requiring years of research and development. This change, which would strand capital and impact parts sharing, is why the advanced engine paths restrict most movement between them. The concept of stranded capital is discussed further in Section II.C.2.f.

TURBO0 is the baseline turbocharged engine and TURBOD is TURBO0 with the addition of cylinder deactivation (DEAC). Chapter 3 of the Draft TSD provides more discussion on engine technologies.

ADEACD is a dual-overhead camshaft engine with advanced cylinder deactivation. Chapter 3 of the Draft TSD provides more discussion on engine technologies.

NHTSA also considers two categories of technology, for model years in which the technology categories are applicable, that the agency could not simulate as part of the CAFE Model's technology pathways. “Off-cycle” and AC efficiency are two types of technologies that improve vehicle fuel economy but are not accounted for using 2-cycle testing. To account for the benefits of these technologies, EPA has allowed manufacturers to generate FCIVs when they add these technologies, which are used to improve a manufacturers' certified fuel economy. As an example, manufacturers can generate FCIVs for technology like active seat ventilation and solar reflective surface coatings that make the cabin of a vehicle more comfortable for the occupants without using less efficient accessories like heat or AC. Instead of including OC and AC efficiency technologies in the technology pathways, NHTSA includes the improvement as a defined benefit that gets applied to a manufacturer's entire fleet in applicable model years instead of to individual vehicles. The defined benefit that each manufacturer receives in the analysis for using OC and AC efficiency technology on their vehicles is located in the Market Data

Input File. Chapter 3.7 of the Draft TSD provides more discussion on how OC and AC efficiency technologies are developed and modeled. Preamble Section VI contains discussion of this program's updates in this rule.

To illustrate how NHTSA simulates technology application, throughout this section NHTSA follows the hypothetical vehicle mentioned above that begins the compliance simulation with a TURBOD engine. The agency's hypothetical vehicle, Generic Motors' Ravine Runner F Series, is a roomy, top-of-the-line sport utility vehicle (SUV). The Ravine Runner F Series starts the compliance simulation with technologies from most technology pathways; specifically, after looking at Generic Motors' website and marketing materials, the agency determines that it has technology that loosely fits within the following technologies that the agency considers in the CAFE Model: it has a turbocharged engine with cylinder deactivation, a fairly advanced 10-speed automatic transmission, a 12V start-stop system, the least advanced tire technology, a fairly aerodynamic vehicle body, and it employs a fairly advanced level of MR. NHTSA tracks the technologies on each vehicle using a “technology key,” which is the string of technology abbreviations for each vehicle. The vehicle technologies and their abbreviations that the agency considers in this analysis are shown in Draft TSD Chapter 2. The technology key for the Ravine Runner F Series is “TURBOD; AT10L2; SS12V; ROLL0; AERO5; MR3.”

b. Defining Manufacturers' Current Technology Positions in the Analysis Fleet

The Market Data Input File is one of four Excel input files that the CAFE Model uses for compliance and effects simulation. The Market Data Input File's “Vehicles” tab (or worksheet) houses one of the most significant compilations of technology inputs and assumptions in the analysis, which is a characterization of the fleet of vehicle models each manufacturer produced for sale in the United States for MY 2024. This provides the starting point from which the CAFE Model adds fuel economy-improving technology. NHTSA calls this fleet the “analysis fleet.” The analysis fleet includes a number of inputs necessary for the Model to add fuel economy-improving technology to each vehicle for the compliance analysis and to calculate the resulting impacts for the effects analysis.

The “Vehicles” tab contains a separate row for each vehicle model. Vehicle models are vehicles that share the same fuel economy value and vehicle footprint. This means that vehicle models with different configurations that affect the vehicle's certification fuel economy value are distinguished in separate rows in the Vehicles tab. For example, the agency's Ravine Runner example vehicle comes in three different configurations—the Ravine Runner FWD, Ravine Runner AWD, and Ravine Runner F Series—which would result in three separate rows.

In each row, NHTSA also designates a vehicle's engine, transmission, and platform codes.

Vehicles that have the same engine, transmission, or platform code are deemed to “share” that component in the CAFE Model. Parts sharing helps manufacturers achieve economies of scale, deploy capital efficiently, and make the most of shared research and development expenses, while still presenting a wide array of consumer choices to the market. The CAFE Model has been developed to treat vehicles, platforms, engines, and transmissions as separate entities, which allows the modeling system to evaluate technology improvements on multiple vehicles that may share a common component concurrently. Sharing also enables realistic propagation, or “inheriting,” of previously applied technologies from an upgraded component down to the vehicle “users” of that component that have not yet realized the benefits of the upgrade. Section 2.1 and Section 4.4 of the CAFE Model Documentation contain additional information about the initial state of the fleet, as well as technology evaluation and inheriting within the CAFE Model.

Each numeric engine, transmission, or platform code designates important information about that vehicle's technology; for example, a vehicle's 6-digit transmission code includes information about the manufacturer, the vehicle's drive configuration (

e.g.,

front-wheel drive, all-wheel drive, 4WD, or rear-wheel drive), transmission type, number of gears (

i.e.,

a 6-speed transmission has 6 gears), and the transmission variant.

Figure II-1 below shows how NHTSA separates the different configurations of the hypothetical Ravine Runner. NHTSA sees by the Platform Codes that these Ravine Runners all share the same platform, but only the Ravine Runner FWD and Ravine Runner AWD share an engine. Even so, all three certification fuel economy values are different, which is common for vehicles that differ in drive type (drive type meaning whether the vehicle has AWD, 4-wheel drive (4WD), front-wheel drive (FWD), or rear-wheel drive (RWD). While it is simpler to aggregate vehicles by model, ensuring that NHTSA captures model variants with different fuel economy values improves the accuracy of the analysis and the potential that estimated costs and benefits from different levels of standards are appropriate. NHTSA includes information about other vehicle technologies at the farthest right side of the Vehicles tab, and in the “Engines,” “Transmissions,” and “Platforms” worksheets, as discussed further below.

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Moving

from left to right on the Vehicles tab, after including general information about vehicles and their compliance fuel economy value, NHTSA includes sales and manufacturer's suggested retail price (MSRP) data, regulatory class information (

e.g.,

domestic passenger automobile, import passenger automobile, or non-passenger automobile), and information about how NHTSA classifies vehicles for the effectiveness and safety analyses. Each of these data points is important to different parts of the compliance and effects analysis, so that the CAFE Model can accurately average the technologies required across a manufacturer's regulatory fleet to meet its CAFE standard or estimate the impacts of higher fuel economy standards on vehicle sales.

Note that not all data columns are shown in this example for brevity.

Next, NHTSA includes vehicle information necessary for applying different types of technology; for example, designating a vehicle's body style allows NHTSA to apply aerodynamic technology appropriately, and designating starting CW values allows the agency to apply MR technology more accurately. Importantly, this section also includes vehicle footprint data, which is needed because NHTSA sets footprint-based standards.

NHTSA also sets product design cycles, which are the years in which the CAFE Model can apply technologies to vehicles. Manufacturers often introduce fuel-saving technologies at a “redesign” of their product or adopt technologies at “refreshes” in between product redesigns. As an example, the redesigned third generation Chevrolet Silverado was released for MY 2019 and featured a new platform, updated drivetrain, increased towing capacity, reduced weight, improved safety, and expanded trim levels, to name a few improvements. For MY 2022, the Chevrolet Silverado received a refresh (or facelift as it is commonly called), with an updated interior, infotainment, and front-end appearance.

Setting these product design cycles provides realistic durations of product stability and ensures that the CAFE Model simulates the opportunities manufacturers have to apply technologies in line with refresh and redesign cycles.

GM Authority, 2022 Chevy Silverado, Last revised: 2022, available at:

https://gmauthority.com/blog/gm/chevrolet/silverado/2022-chevrolet-silverado/

(accessed: Sept. 10, 2025).

During modeling, all improvements from technology application are initially realized on a component and then propagated (or inherited) down to the vehicles that share that component. As such, new component-level technologies are initially evaluated and applied to a platform, engine, or transmission during their respective redesign or refresh years. Any vehicles that share the same redesign or refresh schedule as the component apply these technology improvements during the same model year. The rest of the vehicles inherit technologies from the component during their refresh or redesign year (for engine- and transmission-level technologies) or during a redesign year only (for platform-level technologies). Section 4.4 of the CAFE Model Documentation contains additional information about technology evaluation and inheriting within the CAFE Model.

The CAFE Model also considers the potential safety effect of MR technologies and crash compatibility of

different vehicle types. MR technologies lower the vehicle's CW, which may change crash compatibility and safety, depending on the type of vehicle. NHTSA assigns each vehicle in the Market Data Input File a “safety class” that best aligns with the CAFE Model's analysis of vehicle mass, size, and safety, and include the vehicle's starting CW.

71 72

Vehicle curb weight is the weight of the vehicle with all fluids and components but without the drivers, passengers, or cargo.

NPRM preamble Section II.H.1 and Draft TSD Chapter 7.3 provides more in depth discussion on the impacts of mass reduction on safety.

The CAFE Model includes procedures to consider the direct labor impacts of manufacturers' responses to CAFE regulations, considering the assembly location of vehicles, engines, and transmissions; the percent U.S. content (based on the percent U.S. and Canadian content, as reported by manufacturers to NHTSA); and the dealership employment associated with new vehicle sales. Estimated labor information, by vehicle, is included in the Market Data Input File. Sales volumes included in and adapted from the market data also influence total estimated direct labor projected in the analysis. Chapter 6.2.5 of the Draft TSD contains additional discussion of the labor utilization analysis.

NHTSA then assigns the technologies to individual vehicles. This initial linkage of vehicle technologies is how the CAFE Model knows how to advance a vehicle down each technology pathway. Assigning CAFE Model technologies to individual vehicles is dependent on the mix of information the agency has about any particular vehicle and trends about how a manufacturer has added technology to that vehicle in the past, equations and models that translate real-world technologies to their counterparts in NHTSA's analysis (

e.g.,

drag coefficients and body styles can be used to determine a vehicle's AERO level), and the agency's engineering judgment.

As discussed further below, the agency uses information directly from manufacturers to populate some fields in the Market Data Input File, like vehicle HP ratings and vehicle weight. NHTSA also uses manufacturer data as an input to various other models that calculate how a manufacturer's real-world technology equates to a technology level in the agency's model. For example, the agency calculates initial MR, aerodynamic drag reduction, and ROLL levels by looking at industry-wide trends and calculating—through models or equations—levels of improvement for each technology. The models and algorithms that the agency uses are described further below and in detail in Chapter 3 of the Draft TSD. Other fields, like vehicle refresh and redesign years, are projected forward based on historic trends.

Recall the Ravine Runner F Series example with the technology key “TURBOD; AT10L2, SS12V; ROLL0; AERO5; MR3.” For this example, Generic Motor's publicly available spec sheet for the Ravine Runner F Series says that it uses Generic Motor's Turbo V6 engine with proprietary Adaptive Cylinder Management Engine (ACME) technology. Generic Motor's ACME improves fuel economy and lowers emissions by operating the engine using only three of the engine's cylinders in most conditions and using all six engine cylinders when more power is required. Based on this information, NHTSA would conclude that this engine is turbocharged and uses a form of cylinder deactivation, meaning it would be appropriately classified as TURBOD. Generic Motors uses this engine in several of their vehicles, and the specifications of the engine can be found in the Engines Tab of the Market Data Input File, under a six-digit engine code.

Like the transmission codes discussed above, the engine codes include information identifying the manufacturer, engine displacement (how many liters the engine is), whether the engine is naturally aspirated or force-inducted (turbocharged), and other unique engine attributes.

This is a relatively easy engine to assign based on publicly available specification sheets, but some technologies are more difficult to assign. Manufacturers use different trade names or terms for different technology, and the way that the agency assigns the technology in the agency's analysis may not necessarily line up with how a manufacturer describes the technology. NHTSA must use some engineering judgment to determine how discrete technologies in the market best fit the technology options that the agency considers in the agency's analysis. The agency discusses factors used to assign each vehicle technology in the individual technology subsections below.

In addition to the Vehicles Tab that houses the analysis fleet, the Market Data Input File includes information that affects how the CAFE Model might apply technology to vehicles in the compliance simulation. Specifically, the Market Data Input File's “Manufacturers” tab includes a list of vehicle manufacturers considered in the analysis and several pieces of information about their economic and compliance behaviors. For this analysis, the compliance simulation assumes that manufacturers continue to apply technology to the extent practicable to reach compliance. This modeling change is made by indicating in the “Manufacturers” tab that all manufacturers will comply with NHTSA's standards and is consistent with the recent amendment to EPCA that set civil penalties (

i.e.,

fines) to $0 effective for MY 2022 vehicles and beyond.

The CAFE Model's compliance simulation algorithm is discussed in Section II.C.2.f.

See

Public Law 119-21, 139 Stat. 72, sec. 40006 (July 4, 2025),

https://www.congress.gov/119/plaws/publ21/PLAW-119publ21.pdf.

Finally, NHTSA designates a “payback period” for each manufacturer. The payback period represents an assumption that consumers are willing to buy vehicles with more fuel economy technology because the fuel economy technology saves them money on gas in the long run. For the past several rulemaking analyses using the CAFE Model the agency has assumed that in the absence of CAFE or other regulatory standards, manufacturers apply technology that “pays for itself”—by saving the consumer money on fuel—in 30-months, or 2.5 years. NHTSA has updated the agency's payback period for this proposed rule to assume a full 3-year payback period based on an examination of empirical economics literature. This is discussed in detail in Section II.E.1.a below, and in the Draft TSD and PRIA.

Before the agency begins building the Market Data Input File for any analysis, NHTSA must consider what model year vehicles comprise the analysis fleet. There is an inherent time delay in the data the agency can use for any particular analysis because NHTSA receives compliance data after a model year has been completed.

Using recent data for the analysis fleet is more likely to reflect the current vehicle fleet than older data. Recent data reflects (1) manufacturers' realized decisions on what fuel economy-improving technology to apply; (2) mix shifts in response to consumer preferences; (

e.g.,

more recent data reflects manufacturer and consumer preference towards larger vehicles),

and (3) industry sales volumes that incorporate substantive macroeconomic events. Using an analysis fleet year that

has been impacted by these transitory shocks may not represent trends in future years; however, on balance, updating to using the most complete set of available fleet data provides the most accurate analysis fleet for the CAFE Model to calculate compliance and effects of different levels of future fuel economy standards. Also, using recent data decreases the likelihood that the CAFE Model selects compliance pathways for future standards that affect vehicles already built in previous model years.

See EPA, The 2024 EPA Automotive Trends Report, Greenhouse Gas Emissions, Fuel Economy, and Technology since 1975, EPA-420-R-24-022, pp. 17—21 (2024), available at:

https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P101CUU6.TXT

(accessed: Sept. 10, 2025) (hereinafter, “2024 EPA Automotive Trends Report”).

For example, in this analysis, the CAFE Model must apply technology to the MY 2024 fleet from MYs 2025-2026 for the compliance simulation that begins in MY 2027. While manufacturers have already built MY 2024 and beyond vehicles, the most current, complete dataset with regulatory fuel economy test results to build the analysis fleet at the time of writing remains MY 2024 data for the light-duty fleet.

At the time NHTSA starts building the analysis fleet, data received from vehicle manufacturers

offers the best snapshot of vehicles for sale in the United States in a model year. The mid-model year reports include information about individual vehicles at the vehicle configuration level. NHTSA uses the vehicle configuration, certification fuel economy, sales, regulatory class, and additional technology data from these reports as the starting point to build a “row” (

i.e.,

a vehicle configuration, with all necessary information about the vehicle) in the Market Data Input File's Vehicles Tab. Additional technology data comes from publicly available information, including vehicle specification sheets, manufacturer press releases, owner's manuals, and websites. NHTSA also generates some assumptions in the Market Data Input File for data fields where there is limited data, like refresh and redesign cycles for future model years, and technology levels for certain road load reduction technologies like MR and aerodynamic drag reduction.

49 U.S.C. 32907(a)(2) and 49 CFR part 537.

For this analysis, the light-duty analysis fleet consists of every vehicle model in MY 2024 in nearly every configuration that has a different compliance fuel economy value. This results in nearly 4,000 individual rows in the Vehicles Tab of the Market Data Input File.

The next section discusses how the agency's analysis evaluates how effectively adding technology to a vehicle in the analysis fleet improves that vehicle's fuel economy value.

c. Technology Effectiveness Values

The CAFE Model uses technology effectiveness values to allow it to know which technologies to apply. Without these values, it does not know how effective any particular technology is at improving a vehicle's fuel economy value. Accurate technology effectiveness estimates require information about (1) the vehicle type and size; (2) other technologies on the vehicle or being added to the vehicle at the same time; and (3) and how the vehicle is driven. Any oversimplification of these complex factors could make the effectiveness estimates less accurate.

To build a database of technology effectiveness estimates that includes these factors, NHTSA partners with Argonne. Argonne has developed and maintains a modeling and simulation tool called Autonomie that generates technology effectiveness estimates for the CAFE Model. The Autonomie Model is a mathematical representation of an entire vehicle, including its individual technologies (such as the engine and transmission), overall vehicle characteristics (such as mass and aerodynamic drag), and environmental conditions (such as ambient temperature and barometric pressure). The Autonomie Model simulates vehicle behavior over time.

NHTSA simulates a vehicle model's behavior over the two-cycle tests used to measure vehicle fuel economy.

The two-cycle test is carried out by operating a vehicle on a dynamometer. Using a dynamometer is like running a car on a treadmill following a program—or more specifically, two programs. The programs are the Federal Test Procedure (FTP) and the Highway Fuel Economy Test (HFET). The FTP and HFET are also commonly referred to as the urban cycle and highway cycle, respectively. For the FTP drive cycle, the vehicle meets certain speeds at certain times during the test, or in technical terms, the vehicle must follow a designated speed trace.

The FTP is meant to simulate stop-and-go city driving, and the HFET is meant to simulate steady flowing highway driving at about 50 miles per hour (mph). The agency also uses Society of Automotive Engineers (SAE) recommended practices to simulate hybridized drive cycles,

which involves the test cycles mentioned above as well as additional test cycles to measure battery energy consumption and range. For PHEVs, this analysis utilizes only the gasoline (charge-sustaining) mode for the drive cycles.

NHTSA is statutorily required to use the two-cycle tests to measure vehicle fuel economy in the CAFE program.

See

49 U.S.C. 32904(c) (“Testing and calculation procedures. . . . [T]he Administrator shall use the same procedures for passenger automobiles the Administrator used for model year 1975 (weighted 55 percent urban cycle and 45 percent highway cycle), or procedures that give comparable results.”).

EPA, Emissions Standards Reference Guide: EPA Federal Test Procedure (FTP), Last revised: Mar. 13, 2025, available at:

https://www.epa.gov/emission-standards-reference-guide/epa-federal-test-procedure-ftp

(accessed: Sept. 10, 2025).

SAE, Recommended Practice for Measuring the Exhaust Emissions and Fuel Economy of Hybrid-Electric Vehicles, Including Plug-in Hybrid Vehicles, SAE Standard J1711_202302 (2023), SAE International: Warrendale, PA, available at:

https://www.sae.org/standards/content/j1711_202302/

(accessed: Sept. 10, 2025); SAE, Battery Electric Vehicle Energy Consumption and Range Test Procedure, SAE Standard J1634_202104 (2021), SAE International: Warrendale, PA, available at:

https://www.sae.org/standards/content/j1634_202104/

(accessed: Sept. 10, 2025).

Measuring every vehicle's fuel economy value by using the same test cycles ensures that the fuel economy certification results are repeatable for each vehicle model and comparable across all of the different vehicle models. When performing physical vehicle cycle testing, sophisticated test and measurement equipment is calibrated according to strict industry standards, which ensures repeatability and comparability of the results. Testing variables can include dynamometers, environmental conditions, types and locations of measurement equipment, and precise testing procedures. These physical tests provide the benchmarking empirical data used to develop and verify Autonomie's vehicle control algorithms and simulation results. Autonomie's inputs are discussed in more detail later in this section.

Full-vehicle modeling and simulation are also essential to measuring how all technologies on a vehicle interact. For example, if technology A improves a particular vehicle's fuel economy by 5 percent and technology B improves a particular vehicle's fuel economy by 10 percent, an analysis using single or limited point estimates may erroneously assume that applying both of these technologies together would achieve a simple additive fuel economy improvement of 15 percent. Single point estimates generally do not provide accurate effectiveness values because they do not capture complex relationships among technologies. Technology effectiveness often differs significantly depending on the vehicle type (

e.g.,

sedan or pickup truck) and the way in which the technology interacts with other technologies on the vehicle, as different technologies may provide different incremental levels of fuel economy improvement if implemented alone or in combination with other technologies. Any oversimplification of these complex factors could lead to less accurate technology effectiveness estimates.

In addition, because manufacturers often add several fuel-saving technologies simultaneously when redesigning a vehicle, it is difficult to isolate the effect of adding any one individual technology to the full-vehicle system. Modeling and simulation offer the opportunity to isolate the effects of individual technologies by using a single or small number of initial vehicle configurations and incrementally adding technologies to those configurations. This provides a consistent reference point for the incremental effectiveness estimates for each technology and for combinations of technologies for each vehicle type. Vehicle modeling also reduces the potential for overcounting or undercounting technology effectiveness.

Argonne does not build an individual vehicle model for every single-vehicle configuration in NHTSA's light-duty Market Data Input File. This would be nearly impossible, because Autonomie requires very detailed data on hundreds of different vehicle attributes (

e.g.,

the weight of the vehicle's fuel tank, the weight of the vehicle's transmission housing, the weight of the engine, or the vehicle's 0-60 mph time) to build a vehicle model. For practical reasons, NHTSA cannot acquire 4,000 vehicles and obtain these measurements every time the agency promulgates a new rule, and the agency cannot acquire vehicles that have not yet been built. Rather, Argonne builds a discrete number of vehicle models representative of the most popular vehicles on sale today. The agency refers to the vehicle model's type and performance level as the vehicle's “technology class.” By assigning each vehicle in the Market Data Input File a “technology class,” NHTSA can connect it to the Autonomie effectiveness estimate that best represents how effective the technology would be on the vehicle, accounting for vehicle characteristics like body style (

e.g.,

sedan or pickup truck) and performance metrics. Because each vehicle technology class has unique characteristics, the effectiveness of technologies and combinations of technologies is different for each technology class.

There are 10 technology classes for this analysis: small car (SmallCar), small performance car (SmallCarPerf), medium car (MedCar), medium performance car (MedCarPerf), small SUV (SmallSUV), small performance SUV (SmallSUVPerf), medium SUV (MedSUV), medium performance SUV (MedSUVPerf), pickup truck (Pickup), and high towing pickup truck (PickupHT).

NHTSA uses a two-step process that involves two algorithms to give vehicles a “fit score” that determines which vehicles best fit into each technology class. At the first step, the agency determines the vehicle's size. At the second step, NHTSA determines the vehicle's performance level. Both algorithms consider several metrics about the individual vehicle and compare that vehicle to other vehicles in the analysis fleet. This process is discussed in detail in Draft TSD Chapter 2.2.

Consider NHTSA's example Ravine Runner F Series, which is a medium-sized performance SUV. The exact same combination of technologies on the Ravine Runner F Series operate differently in a compact car or pickup truck because they are different vehicle sizes. The example Ravine Runner F Series also achieves slightly better performance metrics than other medium-sized SUVs in the analysis fleet. By “performance metrics,” the agency means power, acceleration, handling, braking, and so on. For the performance versus standard technology classification, the agency considers the vehicle's estimated 0-60 mph time compared to an average 0-60 mph time for the vehicle's technology class. Accordingly, the “technology class” for the Ravine Runner F Series in the agency's analysis is “MedSUVPerf,” because it meets the criteria of a “performance” 0-60 mph acceleration time.

Table II-2 shows how vehicles in different technology classes that use the exact same fuel economy technology have very different absolute fuel economy values. Note that the Autonomie absolute fuel economy values are not used directly in the CAFE Model; NTHSA calculates the ratio between two Autonomie absolute fuel economy values (one for each technology key for a specific technology class) and applies that ratio to an analysis fleet vehicle's starting fuel economy value.

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Depending on the technology, when two technologies are added to the vehicle together, they may not result in an additive fuel economy improvement. This is an important concept to understand because in Section II.D, NHTSA presents technology effectiveness estimates for every single combination of technology that could be applied to a vehicle. In some cases, technology effectiveness estimates show that a combined technology has a different effectiveness estimate than if the individual technologies were added together individually. However, this is expected and not an error. Continuing NHTSA's example from above, turbocharging technology and dynamic cylinder deactivation (DEAC) technology both improve fuel economy by reducing the engine displacement and accordingly burning less fuel. Turbocharging allows a manufacturer to use a smaller engine that can offer performance equivalent to a larger naturally aspirated engine, and its fuel efficiency improvements are, in part, due to the reduced displacement. DEAC effectively makes an engine with a particular displacement intermittently offer some of the fuel economy benefits of a smaller displacement engine by deactivating cylinders when the work demand does not require the full engine displacement and reactivating them as-needed to meet higher work demands;

the greater the displacement of the deactivated cylinders, the greater the fuel economy benefit. Therefore, a manufacturer upgrading to an engine that uses both a turbocharger and DEAC technology, like the TURBOD engine in the example above, would not see the full combined fuel economy improvement from that specific combination of technologies. Table II-3 shows a vehicle's fuel economy value when using the first-level DEAC technology and when using the first-level turbocharging technology, compared to the agency's example vehicle that uses both of those technologies combined with a TURBOD engine.

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As expected, the percent improvement in Table II-3 between the first and second rows is 1.7 percent and between the third and fourth rows is 0.3 percent, even though the only difference within the two sets of technology keys is the DEAC technology (note that the agency only compares technology keys within the same technology class). This is because there are complex interactions between all fuel economy-improving technologies. The agency models these individual technologies and groups of technologies to reduce the uncertainty and improve the accuracy of the CAFE Model outputs.

Some technology synergies that NHTSA discusses in Section II.D include advanced engine and hybrid powertrain technology synergies. As an example, NHTSA does not see a particularly high effectiveness improvement from applying advanced engines to existing parallel strong hybrid (

e.g.,

P2) architectures.

In this instance, the P2 powertrain improves fuel economy, in part, by allowing the engine to spend more time operating at efficient engine speed and load conditions. This reduces the advantage of adding advanced engine technologies, which also improve fuel economy, by broadening the range of speed and load conditions for the engine to operate at high efficiency. This redundancy in fuel-saving mechanisms results in a lower effectiveness when the technologies are added to each other. Again, NHTSA expects that different combinations of technologies will provide different effectiveness improvements on different vehicle types. These examples all illustrate relationships observed using only full-vehicle modeling and simulation.

A parallel strong hybrid powertrain is fundamentally similar to a conventional powertrain but adds one electric motor to improve efficiency. Draft TSD Chapter 3 shows all of the parallel strong hybrid powertrain options that NHTSA has modeled in this analysis.

Just as NHTSA's CAFE Model analysis requires a large set of technology inputs and assumptions, the Autonomie modeling uses a large set of technology inputs and assumptions. Figure II-2 below shows the suite of fuel consumption input data used in the Autonomie modeling to generate the fuel consumption input data NHTSA uses in the CAFE Model.

EP05DE25.023

As shown in Figure II-2 above, full-vehicle benchmarking is a major source of data for the Autonomie model. For full-vehicle benchmarking, vehicles are instrumented with sensors and tested on both the road and chassis dynamometers (

i.e.,

the full-vehicle treadmills used to exercise the vehicle to provide means to calculate vehicle's fuel economy values) under different conditions and duty-cycles. Vehicles are selected for benchmarking with the goal of selecting a mix of vehicles most representative of vehicle fleet and available technologies, taking into account sales volume, cost, and availability. Some examples of full-vehicle benchmark testing performed in conjunction with the agency's partners at Argonne include a 2019 Chevrolet Silverado, a 2021 Toyota Rav4 Prime, and a 2022 Hyundai Sonata Hybrid.

NHTSA has produced a report for each vehicle benchmarked, which can be found in the docket. As discussed further below, full-vehicle benchmarking data are used as inputs to the engine modeling and Autonomie full-vehicle simulation modeling. Component benchmarking is like full-vehicle benchmarking, but instead of testing a full vehicle, the agency instruments a single production component or prototype component with sensors and tests it on a similar duty-cycle as a full vehicle. Examples of components NHTSA benchmarks include engines, transmissions, axles, electric motors, and batteries. Component benchmarking data are used as an input to component modeling, where a production or prototype component is changed in fit, form, or function and modeled in the same scenario. As an example, NHTSA might model a decrease in the size of holes in fuel injectors to see the fuel atomization impact or see how it affects the fuel spray angle.

For all Argonne full-vehicle benchmarking reports, see Docket No. NHTSA-2023-0022-0010.

NHTSA uses a range of models to do the component modeling. As shown in Figure II-2, battery pack modeling using Argonne's BatPaC Model and engine modeling are two of the most significant component models used to generate data for the Autonomie modeling. NHTSA discusses BatPaC in detail in Section II.D, but briefly, BatPaC is the battery pack modeling tool used to estimate the cost of vehicle battery packs for all hybridized vehicles, which is based on the materials chemistry, battery design, and manufacturing design of the plants manufacturing the battery packs.

Engine modeling is used to generate engine fuel map models that define the fuel consumption rate for an engine equipped with specific technologies when operating over a variety of engine load and engine speed conditions. Some performance metrics captured in engine modeling include power, torque, airflow, volumetric efficiency, fuel consumption, turbocharger performance and matching, pumping losses, and more. Each engine map model has been developed ensuring the engine will still operate under real-world constraints using a suite of other models. Some examples of these models that ensure the engine map models capture real-world operating constraints include simulating heat release through a predictive combustion model, simulating knock characteristics through a kinetic fit knock model,

and using physics-based heat flow and friction models, among others. NHTSA simulates these constraints using data gathered from component benchmarking as well as engineering and physics calculations.

Engine knock occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignited by the spark plug; rather one or more pockets of air/fuel mixture explode outside of the envelope of the normal combustion front. Engine knock can result in unsteady operation and damage to the engine.

IAV develops the engine map models, using their GT-POWER modeling tool, by creating a base, or root, engine map and then modifying that root map, incrementally, to isolate the effects of the added technologies. The engine maps are based on real-world engine

designs. An important feature of the engine maps is that they use a knock model. As noted above, a knock model ensures that any engine size or specification that the agency models in the analysis does not result in engine knock, which could damage engine components in a real-world vehicle. Though the same engine map models are used for all vehicle technology classes, the effectiveness varies based on the characteristics of each class. For example, as discussed above, a compact car with a turbocharged engine has a different effectiveness value than a pickup truck with the same engine technology type. The engine map model development and specifications are discussed further in Chapter 3 of the Draft TSD.

Argonne also compiles a database of vehicle attributes and characteristics reasonably representative of the vehicles in that technology class used to build the vehicle models. Relevant vehicle attributes may include a vehicle's fuel efficiency, HP, 0-60 mph acceleration time, and stopping distance, among others, while vehicle characteristics may include whether the vehicle has all-wheel-drive, 18-inch wheels, summer tires, and so on. Argonne has identified representative vehicle attributes and characteristics for the light-duty fleet from publicly available information and automotive benchmarking databases, such as A2Mac1,

Argonne's Downloadable Dyna

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