NHTSA Tells Truck Engine Makers It Was Never Its Call — a Reset, Not Yet a Rewrite, of Fuel Economy Rules

The National Highway Traffic Safety Administration published an interpretive rule on Aug. 28 concluding that it has never had the statutory authority to set fuel efficiency standards for standalone medium- and heavy-duty truck engines — only for the vehicles those engines sit in. The document, "Resetting NHTSA's Fuel Economy Program: Commercial Medium- and Heavy-Duty On-Highway Vehicles and Work Trucks," is scheduled for Federal Register publication Aug. 31 as document 2026-17756.

The rule itself changes nothing on the ground today. It creates no new compliance obligation and repeals none directly; instead it announces how NHTSA will read its own authority going forward and says the agency will exercise enforcement discretion consistent with that reading while it runs a separate notice-and-comment rulemaking to formally reset the Medium- and Heavy-Duty (MDHD) fuel economy program. Manufacturers building engines to the current Phase 2 standards are not released from those standards by this document alone.

The legal argument

NHTSA's case turns on a narrow reading of the Energy Independence and Security Act of 2007, which the agency says grants it authority over "work trucks and commercial medium-duty or heavy-duty on-highway vehicles" — with no separate mention of engines. The agency argues Congress knew how to regulate engines specifically, because it gave EPA that authority under the Clean Air Act in the same general period, and that giving NHTSA engine-level authority by implication would remove any principled limit on how far down into a truck's components a federal fuel economy standard could reach — tires and transmissions along with engines.

The rule leans on a 2024 Supreme Court decision, cited by NHTSA as Loper Bright Enterprises v. Raimondo, which ended judicial deference to agency interpretations of ambiguous statutes, plus a D.C. Circuit ruling against a separate NHTSA trailer-standard program that the agency reads as rejecting standards for components that aren't complete vehicles. NHTSA Administrator Jonathan Morrison framed the move as returning design choices to manufacturers rather than regulators.

Independent trade coverage of the same document, including reporting from Trucking Dive, confirms the rule also bars NHTSA's medium- and heavy-duty vehicle programs from fining manufacturers for engine-efficiency violations going forward — but adds no additional detail on compliance timing, and no engine maker, EPA, or environmental group had gone on record with a reaction as of this filing.

What actually changes, and when

Nothing is repealed by this document. What NHTSA has done is (1) declare it lacks engine-standard authority, (2) commit to enforcement discretion consistent with that view in the interim, and (3) open a rulemaking to formally rewrite the MDHD program around vehicle-level rather than engine-level standards. That rulemaking will carry its own comment period, and until it closes, the Phase 2 program EPA and NHTSA jointly built — the one NHTSA's own prior estimates put at roughly $170 billion in lifetime fuel savings and a 25% cut in fuel consumption and CO2 for new tractors relative to 2018 baselines — remains EPA's to enforce on the emissions side regardless of what NHTSA does on the fuel-economy side. The two agencies have always run parallel authorities under different statutes; this rule narrows one of them, not both.

What this means for the shipper and forwarder desk

Three things worth tracking, none of them settled by this filing alone:

Equipment costs may loosen, but not yet. If the eventual MDHD rulemaking drops or softens engine-specific standards, engine manufacturers gain flexibility on how they hit fleet-level fuel targets — potentially cheaper compliance paths that show up in new-tractor pricing a model year or two out. Nothing in the interpretive rule changes a truck spec sheet today.

EPA's separate emissions authority is the real backstop. A forwarder assessing counterparty risk in a fleet's long-term cost structure should watch the EPA side of Phase 2, not just NHTSA — engine makers still answer to EPA's greenhouse gas standards regardless of how this rulemaking resolves.

This is round one of a fight, not a final answer. An interpretive rule under this administration is exactly the kind of action likely to draw a legal challenge from state attorneys general or environmental groups once the formal rulemaking is proposed — the same posture that produced litigation over other recent DOT and EPA moves this year. Any fleet planning capital purchases around an assumption of permanently relaxed engine standards is planning around a rule that hasn't been written yet.