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 only by a rechargeable battery.
Cathy Milbourn, a spokeswoman for the EPA, declined to specify a date when the new ratings might be released, saying only that they would come "shortly."
The Volt and Leaf must be rated by the EPA and have those ratings shown on window labels before they are sold.
Both Nissan and GM 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.
''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 that he expected the agency to determine multiple fuel-economy figures for the Volt, based on the distance driven between battery charges.
''Right now it looks like there's going to be a lot on the label," Stephens said. "They're trying to figure out what are all the variables that customers are going to see out there."
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 powertrain, said she expected 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.
''There are going to be new and unique numbers to classify the new and unique behavior of this car," Fletcher 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."
Fletcher said she expected the Volt's window label to at least show a miles-per-gallon equivalency rating for the car when it ran on battery power, as well as a more traditional rating to measure the engine's efficiency after the battery was 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 miles per gallon.
A year ago, GM announced to widespread skepticism that it expected the Volt to earn a city rating of 230 miles per gallon 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 miles per gallon in city driving.) The EPA later rejected the formula that resulted in the two estimates.
The Volt became a source of debate this week among some automotive reviewers who test-drove it. Some accused GM of lying about the Volt's design after the company revealed that in some situations — such as at high speeds after the battery has been drained — the gas engine helped propel the car's wheels, much like a traditional hybrid car.
For years, GM described the Volt as an electric car in which gasoline powers a generator, not the wheels.
Stephens said GM had not previously disclosed that detail for competitive reasons until it received a patent on the design, which happened in September.
The questions raised about the Volt's hybrid credentials are not expected to weigh heavily on potential buyers, said one auto analyst who attended the test-drive in Detroit.
''It might matter to some techno-geeks, but not to anybody else," said Joseph Phillippi of the firm Auto Trends Consulting in Short Hills, N.J.
The EPA has already weighed in on that topic. Its website classifies the Volt as a "plug-in hybrid" while calling the Leaf simply an "electric car."