DETROIT  About two months before two new plug-in cars go on sale in the United States, the federal government is struggling with how to rate the fuel economy of mass-market plug-in vehicles.

How the Environmental Protection Agency rates the two cars, the Chevrolet Volt and Nissan Leaf, could have a big influence on consumers’ perceptions of vehicles that run on electricity. General Motors, which makes the Volt, and Nissan are anxiously awaiting the agency’s decision as they start production of the cars and complete marketing plans for rollouts in December.

Providing the customary city and highway miles-per-gallon information would make little sense for the Volt, which can drive 25 to 50 miles on battery power before its gas engine kicks on, and even less so for the Leaf, which is powered by only a rechargeable battery.

Cathy Milbourn, a spokeswoman for the E.P.A., declined to specify a date when the new ratings might be released, saying only that they would come “shortly.”