DETROIT — Just two months before two new battery-powered cars go on sale in the United States, the Environmental Protection Agency is struggling with how to rate the fuel economy of mass-market plug-in vehicles.
How the EPA 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, an EPA spokeswoman, declined to specify a date when the new ratings might be released, saying only that they would come shortly.
Both Nissan and GM are in discussions with the agency about what the fuel economy information, which is posted on the window stickers of new vehicles, will state, company officials said.
"We don't have an official position on what they should do," said Brian Brockman, a Nissan spokesman. "We expect there will be some form of 'equivalency rating,' like how many miles the Leaf can get per the number of kilowatt hours charged."
Thomas G. Stephens, GM's vice chairman for global product operations, said he expected the agency to determine multiple fuel-economy figures for the Volt, based on the distance driven between the vehicle's battery charges.
"Right now, it looks like there's going to be a lot on the label," Stephens said.
The EPA has proposed changing the labels on all new cars, including the possibility of assigning an overall letter grade, in part to address the issue of electric and hybrid vehicles.
But the proposal would not take effect until the automakers' 2012 models. In the meantime, Milbourn said, the agency was trying to determine "the appropriate information that will go on the 2011 model year label."
Milbourn said the agency would use its standard highway and city testing procedures on the Volt. Since 2008, the agency has been using a battery of five test drives that cover a total distance of 43.9 miles.
Pam Fletcher, GM's chief engineer for the Volt's power train, said she expects multiple tests to capture ratings with the battery in various states of charge.
The testing takes two days for a typical vehicle but seven days for the Volt, she said.
"We need to talk about electricity usage, and we need to talk about gasoline usage, and we need to figure out the best way to do that," she said.
Fletcher said she expects the Volt's window label to at least show an mpg equivalency rating for the car when it runs on battery power, as well as a more traditional rating to measure the engine's efficiency after the battery is drained. In the latter situation, the car should get "some kind of combined fuel economy that's in the mid to upper 30s," she said.
GM 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 mpg.
A year ago, GM announced to widespread skepticism that it expected the Volt to earn a city rating of 230 mpg based on a draft proposal for new testing procedures. Nissan later calculated the Leaf, using the same method, at 367 mpg. (The vehicle with the highest EPA rating is the Toyota Prius, with 51 mpg in city driving.)
The EPA later rejected the formula that resulted in the two estimates.