Products Liability Law Daily Wrap Up, INTERPRETIVE RULES—MOTOR VEHICLES—NHTSA says its fuel economy authority covers vehicles, not engines, (Aug 31, 2026)

By WK Editorial Staff

The Energy Independence and Security Act did not direct the agency to set fuel economy standards for component parts.

The National Highway Traffic Safety Administration (NHTSA) issued an interpretive rule concluding that its authority under the Energy Independence and Security Act of 2007 (EISA) extends to establishing fuel economy standards for commercial medium- and heavy-duty on-highway vehicles and work trucks, but does not authorize separate fuel economy standards for engines. The agency said it will review existing medium- and heavy-duty (MDHD) fuel economy regulations in a future rulemaking to determine whether they are consistent with this interpretation and, in the meantime, will exercise its enforcement authority in accordance with the new interpretation (NHTSA Interpretive rule, 91 Fed. Reg. 55788 (Aug. 31, 2026)).

Statutory language. The interpretive rule centers on EISA’s direction that NHTSA establish a fuel efficiency improvement program for commercial medium- and heavy-duty on-highway vehicles and work trucks. NHTSA said the statute repeatedly refers to “vehicles” and authorizes separate standards for different classes of vehicles, but does not authorize standards for engines on a standalone basis. The agency noted that EISA defines both commercial medium- and heavy-duty vehicles and work trucks as categories of vehicles and requires NHTSA to develop testing procedures, metrics, compliance protocols, and standards for those vehicles.

According to NHTSA, Congress demonstrated in other statutes that it knows how to distinguish between vehicles and engines when granting regulatory authority. The agency contrasted EISA with the Clean Air Act, which expressly authorizes the Environmental Protection Agency (EPA) to regulate both motor vehicles and motor vehicle engines. NHTSA said the absence of comparable language in EISA indicates that Congress limited the agency’s authority to vehicle standards.

Phase 1 and Phase 2 rules. The rule reviews the history of NHTSA’s MDHD program, including the Phase 1 rule issued in 2011 and the Phase 2 rule issued in 2016. In both rulemakings, NHTSA established separate standards for certain engines and justified that approach by interpreting its authority broadly. The agency previously argued that its authority to create separate standards for different classes of vehicles, together with its responsibility to achieve maximum feasible fuel efficiency improvements, supported standalone engine standards.

NHTSA now states that those earlier interpretations extended beyond the best reading of EISA. The agency concluded that the authority to regulate vehicles does not inherently include authority to regulate individual vehicle components, including engines.

Court decisions. The agency also relied on judicial decisions addressing the limits of federal regulatory authority. NHTSA pointed to the U.S. Court of Appeals for the D.C. Circuit’s decision in Truck Trailer Manufacturers Association v. EPA, which held that NHTSA lacked authority to establish fuel economy standards for trailers because trailers are not vehicles that consume fuel. NHTSA said the reasoning in that case supports its conclusion that an engine, while affecting fuel economy, is not itself a vehicle and therefore cannot be regulated through separate fuel economy standards under EISA.

The rule further cites the Supreme Court’s decision in Loper Bright Enterprises v. Raimondo, which emphasized that agencies must adopt the best reading of a statute rather than rely on a merely permissible interpretation. NHTSA said the best reading of EISA is that Congress authorized vehicle standards only.

Next steps. NHTSA stated that the interpretive rule does not amend existing regulations or change current compliance obligations. Instead, it establishes the agency’s interpretation of the law and lays the foundation for future notice-and-comment rulemaking to reset the MDHD fuel economy program. The agency said it will consider revisions necessary to align the program with its interpretation and will evaluate whether changes to existing vehicle standards are warranted as part of that process.

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