Fuel Efficiency Rule for Truck Engines Faces NHTSA Reset

The National Highway Traffic Safety Administration is changing how it interprets federal fuel efficiency law for medium- and heavy-duty trucks.

NHTSA announced on August 28 that it no longer believes federal law allows the agency to set separate fuel efficiency standards for standalone truck engines. Under its new interpretation, NHTSA can regulate complete commercial vehicles but not individual components.

The action does not immediately cancel the existing standards. It also does not create new requirements for truck drivers or change the rules for trucks already on the road.

Instead, the decision begins a longer process to reconsider the federal fuel efficiency program for medium- and heavy-duty vehicles. NHTSA will propose specific regulatory changes through a separate rulemaking.

How the Fuel Efficiency Program Started

Congress established the commercial vehicle fuel efficiency program through the Energy Independence and Security Act of 2007.

The law directed NHTSA to develop standards for commercial medium- and heavy-duty vehicles. The program covers on-highway commercial vehicles with a gross vehicle weight rating of at least 10,000 pounds.

It also covers certain work trucks. These are vehicles rated between 8,500 and 10,000 pounds that are not classified as medium-duty passenger vehicles.

NHTSA and the EPA issued the first phase of the program in September 2011. The agencies created standards for different classes of commercial vehicles. They also established separate standards for engines used in combination tractors and vocational vehicles.

The Phase 1 vehicle standards generally became mandatory with model year 2016 vehicles. Mandatory engine standards began with model year 2017.

NHTSA and EPA finalized a second phase in October 2016. The Phase 2 rule increased the standards for vehicles and engines covered by the original program. NHTSA’s Phase 2 requirements generally applied to model year 2021 and newer vehicles.

Until now, NHTSA interpreted the 2007 law as allowing it to regulate both complete commercial vehicles and certain truck engines separately.

Why NHTSA Changed Its Fuel Efficiency Interpretation

NHTSA now says the Energy Independence and Security Act only authorizes the agency to establish fuel efficiency standards for complete vehicles.

According to NHTSA, the law does not specifically give the agency authority to regulate a truck engine as a separate component. The agency said the same limit applies to transmissions, tires, and other individual parts.

NHTSA compared its authority with the authority Congress gave EPA under the Clean Air Act. That law specifically addresses EPA’s authority over motor vehicles and engines. NHTSA’s authority comes from a different law focused on commercial vehicle fuel efficiency.

The agency also cited federal court decisions supporting its new interpretation.

One was a 2021 decision involving federal standards for trailers. The U.S. Court of Appeals for the District of Columbia Circuit found that NHTSA had exceeded its authority by treating trailers as vehicles covered by the fuel efficiency program.

NHTSA said the court’s reasoning also applies to engines. According to the agency, authority over a complete vehicle does not automatically provide authority over every component inside it.

The agency also cited the Supreme Court’s 2024 decision in Loper Bright Enterprises v. Raimondo. That ruling changed how courts review federal agencies’ interpretations of unclear laws.

Based on those decisions and its latest review, NHTSA now believes separate truck engine standards fall outside its legal authority.

Current Fuel Efficiency Standards Remain in Place

The new action is an interpretive rule. It explains how NHTSA currently understands its authority under federal law.

The official Federal Register document is scheduled for publication on August 31, 2026. It is listed under docket number NHTSA-2026-1948.

Importantly, the document states that the new interpretation does not change the existing standards by itself. It also does not immediately alter the rights or responsibilities established under the current medium- and heavy-duty vehicle program.

Therefore, the announcement is not the final removal of the truck engine standards. NHTSA must complete another rulemaking before formally revising its regulations.

NHTSA said it will use its enforcement authority in a way that follows the new interpretation while that process is underway. The document does not provide more details about how that enforcement approach will work.

The agency also assigned no immediate costs or benefits to the interpretive rule. NHTSA will study possible costs and benefits when it proposes specific regulatory changes.

What Happens Next With Fuel Efficiency Rules

NHTSA’s next step will be a separate notice-and-comment rulemaking.

During that process, the agency will review the current medium- and heavy-duty fuel efficiency program. It will consider how to revise standards that do not match its new interpretation.

NHTSA said it will reconsider the separate truck engine standards. It will also examine how vehicle manufacturers can comply with the program when engines are not regulated separately.

The agency may review other parts of the vehicle standards. The Federal Register document says NHTSA will consider whether adjustments to those standards are needed. However, it does not identify every requirement that could change.

A proposed rule should provide the specific revisions being considered. The public comment period will allow manufacturers, trucking organizations, environmental groups, and other interested parties to respond.

After reviewing those comments, NHTSA could publish a final rule. The agency has not provided a date for completing the process.

What the Action Means for Truck Drivers

The NHTSA announcement does not require drivers to modify their trucks. It does not establish a new roadside inspection item or direct drivers to replace engines or other equipment.

The action mainly concerns NHTSA’s authority over manufacturers and the agency’s method for setting fuel efficiency standards.

NHTSA argues that a vehicle-based system will give manufacturers more flexibility. A manufacturer could improve overall vehicle efficiency through engines, transmissions, cab designs, tires, or other parts.

The agency also said the change could lower commercial truck prices. However, neither official document provides an estimated price reduction. Any effect on truck prices will depend on the specific changes included in the future rule.

For now, the main development is NHTSA’s new legal position. The existing fuel efficiency standards have not been formally rewritten. The full effect will become clearer when the agency publishes its proposed changes.