The National Highway Traffic Safety Administration has concluded that it does not have the authority to set fuel economy standards for heavy-duty engines, reversing the position the agency took when it built the program in 2011 and starting the process of rewriting the federal fuel efficiency rules that apply to commercial trucks.
The interpretive rule published August 31, and took effect the same day, specifically 91 FR 55788 under docket NHTSA-2026-1948. Interpretive rules are not subject to notice-and-comment requirements under the Administrative Procedure Act, so it carries no comment period.
The distinction NHTSA draws is between a vehicle and its components. The Energy Independence and Security Act of 2007 directs the agency to prescribe separate average fuel economy standards for work trucks and commercial medium-duty or heavy-duty on-highway vehicles at 49 U.S.C. 32902(b)(1)(C), and to establish a fuel efficiency improvement program for those vehicles at 32902(k). NHTSA now reads that language as reaching vehicles only: “[N]owhere in NHTSA’s standards-setting authority does Congress instruct NHTSA to set fuel economy standards for component parts,” the rule states.
The agency puts the point plainly: “an engine is not a vehicle.” It continues, “A vehicle’s engine necessarily impacts its fuel economy. So do tires, transmissions, and tailpipes. However, that does not grant NHTSA authority to set fuel economy standards for those components.”
Much of the reasoning turns on a comparison with EPA. The Clean Air Act expressly authorizes EPA to regulate classes of new motor vehicles or new motor vehicle engines. EISA contains no equivalent grant for NHTSA, which the agency treats as intentional. Congress “understands the difference between vehicles and engines, and that when it wants an agency to have authority to regulate one or both, it says so,” the rule states, adding that “a desire by the agencies to achieve regulatory harmonization does not empower NHTSA to expand its statutory authority.”
Nothing is repealed by the document itself. The standards under review sit in 49 CFR parts 523, 534, and 535, and include the separate standards that the Phase 1 rule in 2011 and the Phase 2 rule in 2016 set for combination tractors and vocational vehicles. Work trucks, as EISA defines them, are rated between 8,500 and 10,000 pounds gross vehicle weight. NHTSA said it will revise the program through a separate notice-and-comment rulemaking, and that in the meantime, “pending the rulemaking process, NHTSA will exercise its enforcement authority with regard to affected standards in accordance with the interpretation set forth in this rule.”
The agency identified what that rulemaking will have to work out by reconsidering the engine standards, ensuring that manufacturers have practicable means to comply in the absence of separate engine standards, considering adjustments to the vehicle standards as warranted, and examining the relationship between EPA’s 2023 nitrogen oxide standards taking effect in model year 2027 and fuel consumption. No schedule was announced.
The action lands in a federal picture that has narrowed twice already this year. EPA’s rescission of the 2009 endangerment finding, published in February and effective April 20, removed the greenhouse gas standards for light-, medium-, and heavy-duty vehicles that were built on it, reaching heavy-duty Phase 2 and Phase 3. EPA’s proposed amendments to the model year 2027 and later heavy-duty NOx program, which drew comments through August 29, would revise warranty, useful life, and diesel exhaust fluid inducement requirements. That criteria pollutant program is the federal standard engine makers are building model year 2027 products to meet.
NHTSA said the interpretive rule carries no independent costs or benefits and that those will be assessed in the subsequent rulemaking, for which the agency has not announced a date.
