EPA Proposes Two-Year Delay of Biden-Era Vehicle Emissions Standards
On May 14, 2026, the U.S. Environmental Protection Agency (EPA) announced that it is reconsidering the criteria pollutant standards for new light- and medium-duty vehicles. The first phase of EPA’s proposal would push back the phase-in schedule of the “Tier 4” criteria pollutant standards from model year 2027 to model year 2029, giving automakers two additional model years to comply with the existing Tier 3 framework before the more stringent requirements begin to apply—and giving EPA time to conduct a second rulemaking to amend the Tier 4 emissions standards.
The Biden-Era Rules
In 2024, EPA finalized its “Multi-Pollutant Emissions Standards for Model Years 2027 and Later Light-Duty and Medium-Duty Vehicles.” That rule established more stringent emissions requirements for vehicles beginning with model year 2027.
The criteria pollutant portion of the 2024 rule, often referred to as the Tier 4 program, addressed pollutants such as non-methane organic gases, nitrogen oxides, particulate matter, carbon monoxide, and formaldehyde. It also included changes to certification testing and other compliance requirements. Tier 4 built on EPA’s existing Tier 3 program, which began applying to model year 2017 vehicles.
The 2026 Proposed Rule[1]
EPA is proposing to delay the Tier 4 phase-in for model years 2027 and 2028. Instead of beginning in model year 2027, the Tier 4 requirements would kick in for model year 2029. During the delay period, manufacturers would continue to comply with the existing Tier 3 standards.
EPA’s Basis for Delaying Compliance with Tier 4
EPA’s latest proposal is centered around the agency’s view that the assumptions underlying the 2024 rule have changed. The 2024 rule was developed with the expectation of higher electric vehicle (EV) adoption. Because EVs have no tailpipe emissions, higher EV volumes were expected to help manufacturers meet fleet-average emissions standards—meaning that more stringent limitations for internal combustion engines would be possible to achieve at scale.
EPA now says those assumptions no longer reflect current market conditions: “In the intervening years since the Biden-Harris Administration established the 2024 Tier 4 emission standards, the assumptions about the trajectory of EVs have not come true. This makes the Tier 4 standards unattainable for manufacturers and drives up the cost of vehicles as they try to comply.”[2]
What This Means for Industry
For automakers, the proposal would provide additional time to prepare for Tier 4 compliance (when—or if—Tier 4 takes effect). EPA estimates that the proposed delay will reduce compliance costs by approximately $1.66 to $1.77 billion.
EPA has requested input on the proposed rule and will accept comments through July 6. Any final rule delaying Tier 4 implementation is likely to be challenged in litigation. Notably, in two other delay-rule cases, the D.C. Circuit vacated EPA’s stay of the methane NSPS OOOOa rule and delay of the Risk Management Program amendments.[3]
The delay in the Tier 4 phase-in, if finalized, provides EPA time to conduct a second rulemaking to amend the emissions standards, and the record developed in this rulemaking may shape that later Part 2 phase.
[1] 91 Fed. Reg. 28,463 (May 18, 2026).
[2] EPA, EPA Proposes to Delay Unattainable Biden-era Vehicle Standards, Projecting $1.7 Billion in Savings (March 14, 2026) (here).
[3] See Clean Air Council v. Pruitt, 862 F.3d 1 (D.C. Cir. 2017); Air All. Houston v. Env’t Prot. Agency, 906 F.3d 1049 (D.C. Cir. 2018).
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