"An Engine Is Not a Vehicle": NHTSA Says It Never Had the Power to Set Truck-Engine Fuel Standards — and Will Rewrite the Whole Heavy-Duty Program

Since 2016 a new Class 8 tractor has had to clear two federal fuel-efficiency bars: one for the whole truck and a separate one for its engine. On Monday, August 31, the National Highway Traffic Safety Administration said the second bar should never have existed. In an interpretive rule signed by Administrator Jonathan Morrison, the agency concluded that the Energy Independence and Security Act of 2007 authorizes it to set fuel-economy standards for complete commercial vehicles — and for nothing smaller. "An engine is not a vehicle," the rule says. Then it adds the part that matters for anyone ordering trucks: the existing standards are not repealed, not yet, and the fix will come through a full rewrite of the program.

What actually happened on August 31

The document is an interpretive rule, not a regulation. It went into effect the day it was published, with no comment period, because it changes how NHTSA reads the statute rather than the text of 49 CFR parts 523, 534 and 535. The agency's reasoning leans on two court decisions. In 2021 the D.C. Circuit threw out the Phase 2 fuel-efficiency standards for trailers, holding that a trailer with no motor is not a "vehicle" that uses fuel. In 2024 the Supreme Court's Loper Bright decision ended the practice of courts deferring to an agency's reading of an ambiguous law. Put together, NHTSA now argues that if it could regulate an engine on its own it could equally regulate tires, transmissions and tailpipes — and that Congress never handed it that reach.

The second shoe: the vehicle standards may move too

The engine standard is the headline, but the rule says NHTSA will also "consider whether any adjustments to the vehicle standards are warranted." That is the sentence fleets should read twice. Under a vehicle-only approach a manufacturer could hit a tractor's target through aerodynamics, axle ratios, transmissions, tires or idle reduction rather than through the engine map — and if the targets themselves are loosened, the truck that arrives in 2028 may be specified quite differently from the one on order today. One item the agency names for review is the trade-off between the 2027 NOx aftertreatment and fuel consumption, which engine makers have argued costs efficiency.

This is the second time in fifteen months NHTSA has cut into the same program. In June 2025 it issued a similar interpretive rule on passenger-car CAFE that also, almost in passing, said the agency had no authority to fine violators of the heavy-duty standards, that its credit-trading scheme for trucks was improper, and that it had leaned on electric vehicles in setting the targets without a mandate to do so. Monday's rule cites that document as the starting point for the reset.

“ The Trump Administration is getting out of the way so manufacturers can decide how they want to meet fuel efficiency requirements. ”

Why it is now the only rule left

The Phase 2 program was a joint EPA–NHTSA rule adopted in October 2016, covering model years 2021 through 2027 and sold on a promise of about $170 billion in lifetime fuel savings. The EPA half — greenhouse-gas standards for trucks and engines — was repealed in February 2026 when the agency rescinded its 2009 endangerment finding and with it every vehicle GHG standard, effective April 20. That leaves NHTSA's fuel-consumption standards as the only federal efficiency requirement on a heavy truck. Whatever the agency writes in the reset will be the whole rulebook, not one layer of it. The administration says the change will bring truck prices down; neither the rule nor the announcement puts a number on it.

Who said what

The Truck and Engine Manufacturers Association, which has lobbied against the engine layer for years, welcomed the move. The group said it looks forward to working with NHTSA on standards "that reflect real-world customer adoption of fuel-saving technologies" and warned that "without modification, the current engine and vehicle standards will considerably restrict engine and vehicle specification choices of our customers." On the other side, the Sierra Club's Katherine García said in a statement reported by Land Line that weakening truck efficiency standards "means locking truckers and businesses into greater dependence on diesel and greater exposure to volatile fuel prices." No individual engine maker — Cummins, Daimler, Volvo or PACCAR — had commented on the record by Tuesday, and the American Trucking Associations had not issued a statement.