The U.S. Energy Department (DOE) on Monday proposed reducing electric vehicles’ (EV) mileage ratings to meet government fuel economy requirements, a move that could force automakers to sell more low-emissions cars or improve conventional models.

DOE wants to significantly revise how it calculates the petroleum-equivalent fuel economy rating for electric and plug-in electric hybrids for use in the National Highway Traffic Safety Administration’s (NHTSA) Corporate Average Fuel Economy (CAFE) program.

The current system has not been updated in more than two decades.

“Encouraging adoption of EVs can reduce petroleum consumption but giving too much credit for that adoption can lead to increased net petroleum use because it enables lower fuel economy among conventional vehicles, which represent by far the majority of vehicles sold,” DOE said in its proposed regulation.