"It's a new process. No one has done a vehicle like this before," Posawatz said. "We would like to have 80% of the people get better than the label."
A vehicle of the Volt's design should be able to exceed 100 mpg in tests, said Michael Duoba, a research engineer at Argonne National Laboratory in Argonne, Ill., and chairman of the Society of Automotive Engineers committee trying to develop fuel-economy tests for plug-in cars. Argonne is testing models that use similar technology to make its assessment.
Depending on assumptions about how much gasoline is consumed after the battery loses its charge on the road, the Volt could get 120 mpg to 200 mpg, he said. Modified Prius models, with an electric range of about 10 miles, may have difficulty beating 100 mpg in the same tests, he said.
Toyota may launch a plug-in Prius model for 2010 with an all-electric range of at least 10 miles, spokesman John Hanson said. Chrysler plans its own plug-in electric car for 2010.
"It's too early to say what the overall miles-per-gallon figure is going to be" on the plug-in Prius, Hanson said.
Some experts say the miles-per-gallon measure needs rethinking as automakers move toward electric vehicles.
"Reliance on an mpg standard alone will soon be outdated and will not accurately reflect the need for higher fuel efficiency," Don Foley, executive director of the Progressive Automotive X Prize, said Friday in a statement.
The contest is a $10-million competition to "design, build and bring to market 100-mpge [miles per gallon equivalent] vehicles that people want to buy and that meet market needs for price, size, capability, safety and performance."
Organizers of the X Prize have proposed mpge as a new measure of efficiency. Mpge expresses fuel economy in terms of the energy content of a gallon of gasoline. Energy delivered via electrical cord would be converted to the number of gallons of gasoline containing an equivalent amount of energy in order to calculate mpge.
GM, at least so far, is not participating in the competition.
