NHTSA Issues Interpretive Rule on Medium-and Heavy-Duty Engine Efficiency Standards

On August 31, 2026, the National Highway Traffic Safety Administration's (NHTSA) issued an interpretive rule declaring the agency lacks the statutory authority to mandate standalone fuel-efficiency standards on medium-and heavy-duty engines. Because several motorhomes utilize medium- and heavy-duty chassis, the forthcoming rulemaking could have implications for motorhome chassis and powertrain manufacturers. 

For over a decade, motorized RV manufacturers have been required to operate within the regulatory framework created by joint Environmental Protection Agency (EPA) and NHTSA greenhouse gas and fuel-efficiency programs. Those programs included separate fuel-efficiency standards for certain medium- and heavy-duty engines, in addition to standards for complete vehicles. The current regulations mandate that diesel and gasoline engine manufacturers meet certain efficiency baselines at the individual component level. Under the new Federal Register notice, NHTSA has concluded that its legal authority under the Energy Independence and Security Act (EISA) is limited to regulating the fuel efficiency of complete vehicles, not standalone engines or vehicle subcomponents

Shifting the Impact From Powertrain to Design

Heavy-duty engines and chassis are designed to serve a range of vehicle applications, including motorhomes with distinct performance, torque, weight, and durability requirements. By lifting the standalone engine restrictions, NHTSA states that manufacturers would have greater flexibility to innovate and tailor designs to meet the real-world operational demands of their customers. For motorhome chassis manufacturers, that flexibility could allow a broader range of approaches to meeting vehicle-level fuel-efficiency requirements. 

However, NHTSA’s interpretation itself does not change existing standards. Instead, it establishes the legal foundation for a future rulemaking to reset NHTSA’s medium- and heavy-duty fuel-efficiency program. Until then, NHTSA has stated it will exercise its enforcement authority in alignment with this new interpretive rule.

Because EPA retains separate legal authority to regulate emissions from motor vehicles and engines under the Clean Air Act, RV manufacturers will still need to monitor how the EPA coordinates its upcoming emissions standards with NHTSA's revised, vehicle-centric framework. 

The RV Industry Association will continue to monitor this issue as NHTSA moves forward with its future rulemaking.  For more information about this issue, please contact the Association’s Senior Manager of Regulatory Affairs, Bill Erny, at berny@rvia.org