NHTSA Moves to Rewrite Truck Fuel-Economy Rules The National Highway Traffic Safety Administration says it lacks authority to regulate medium- and heavy-duty truck engines separately from complete vehicles, but the existing standards have not yet been eliminated. - The National Highway Traffic Safety Administration (NHTSA) is reconsidering fuel-economy rules for trucks. - NHTSA states it cannot regulate medium- and heavy-duty truck engines independently from the vehicles themselves. - Current standards for truck fuel-efficiency remain in place despite review efforts. *Summarized by AI Federal fuel-efficiency standards for medium- and heavy-duty truck engines are headed for elimination under a new legal interpretation from the National Highway Traffic Safety Administration. But despite the Trump administration’s characterization of the action as reversing an Obama-era rule, the existing engine standards did not disappear August 31. Instead, NHTSA issued an interpretive rule concluding that Congress authorized it to regulate the fuel efficiency of complete commercial vehicles, but not engines as separate components. The agency plans to revise its medium- and heavy-duty fuel-efficiency program through a future notice-and-comment rulemaking. For fleets ordering 2027 trucks, nothing will change immediately. The trucks and engines being introduced for the Environmental Protection Agency’s 2027 nitrogen-oxide emissions requirements are still coming. However, the federal fuel-efficiency framework that manufacturers will face in the future has become less certain. Engine Standards Remain on the Books NHTSA’s current Phase 2 regulations establish fuel-consumption standards at two levels for most heavy-duty trucks: one for the complete tractor or vocational vehicle and a separate standard for the engine. Those standards include requirements for model-year 2027 and later trucks and engines. NHTSA now says the Energy Independence and Security Act of 2007 did not give it the legal authority to regulate an engine independently from the vehicle in which it is installed. “An engine is not a vehicle,”