Info Trends 24

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.

The Volt and Leaf must be rated by the E.P.A. and have those ratings shown on window labels before they are sold.

Both Nissan and G.M. are in discussions with the agency about what the fuel economy information on the window stickers of new vehicles will state, company officials said. But they said they were in the dark about the outcome and its timing.

G.M. has said the Volt, which has a 9.3-gallon gas tank, would have about a 310-mile range on a depleted battery, which calculates to 33.3 miles per gallon.

For years, G.M. described the Volt as an electric car in which gasoline powers a generator, not the wheels.

The E.P.A. has already weighed in on that topic. Its Web site classifies the Volt as a “plug-in hybrid” while calling the Leaf simply an “electric car.”