That would give the Volt, which is expected to arrive in showrooms late next year, by far the highest fuel efficiency rating of any car now rated by the Environmental Protection Agency. The current EPA mileage leader is the Toyota Prius hybrid, which is rated at 51 mpg in city driving.
The Volt is designed to run on electric power only for about 40 miles, after which a small gasoline engine kicks in to recharge the battery, giving it a total range of more than 300 miles. The battery can be recharged by plugging in to a home outlet.
GM's estimated mileage rating for the Volt is based on city driving, where it can take full advantage of its all-electric capability. Highway mileage would be lower because it would require more work from the gasoline engine.
"From the data we've seen, many Chevy Volt drivers may be able to be in pure electric mode on a daily basis without having to use any gas," GM Chief Executive Fritz Henderson said Tuesday.
The automaker's fuel economy estimate hasn't been confirmed by the EPA, which is developing a new methodology for calculating fuel economy ratings for cars that can travel significant distances powered only by electricity. GM said it used the EPA's preliminary guidelines in developing its mileage estimates for the Volt.
The EPA publishes mileage estimates for vehicles sold in the U.S. based on city and highway driving, as well as a combined city-highway mileage estimate.
GM said it had calculated a highway mileage estimate for the Volt but didn't release the figure. The automaker said it was confident the car's combined city-highway fuel economy "will be in the triple digits."
The mileage estimate doesn't include the electricity needed to charge the Volt's nearly 400-pound lithium-ion battery, causing some analysts to question the usefulness of the number.
"EPA fuel-consumption measures are really inappropriate for the Volt," said Kevin Smith, editorial director for online automotive site Edmunds.com. "The EPA doesn't measure the energy consumed when charging the car via plug-in, and depending on your driving, that may be all the energy it needs."
That could include a lot of potential drivers. Scott Samuelson, director of the National Fuel Cell Research Center at UC Irvine, estimates that 60% of Southern Californians drive less than the Volt's 40-mile all-electric range each day.
GM said it expected the Volt to consume as little as 25 kilowatt-hours of electricity per 100 miles of city driving, or about 3 cents a mile, based on the average cost of electricity in the United States of 11 cents per kilowatt-hour. (Many Californians pay more. Southern California Edison's residential customers, for example, pay 15.5 cents per kilowatt hour, according to state regulators.)
Electric-vehicle proponents, although encouraged by the Volt's preliminary numbers, cautioned GM not to overstate the car's capabilities.
"The phrase 'Your mileage may vary' is particularly true with plug-in hybrids, because how you drive will have a significant impact on how much gasoline the vehicle consumes," said Jim Kliesch, a senior engineer with the Union of Concerned Scientists.
"It would be a real shame if such a promising technology got a bad rap because they raised expectations too high," Kliesch said.
GM said it had produced about 30 Volts and was making 10 a week at its pre-production facility in Warren, Mich. GM is expected to announce this week that the Volt's batteries will be assembled at a plant in the Detroit area. The production version car will be built at GM's Detroit-Hamtramck assembly plant.
As the pre-production vehicles are road-tested, estimates of the cars' all-electric range and their eventual fuel economy rating could change. Volts currently are being road-tested in Yuma, Ariz., to gauge their performance in hot weather conditions.
GM has staked much of its technological reputation on the Volt and has touted the vehicle relentlessly for more than two years. Some critics have derided the vehicle as an expensive piece of "vaporware" that would never achieve significant sales, but the automaker has consistently maintained that it is committed to the Volt.
"Right now, there are no foreseeable roadblocks to the program and the program's timing," GM spokesman Rob Peterson said.
Although GM has not released pricing information on the Volt, industry analysts estimate it will cost about $40,000, though the automaker says the car's list price would probably be lowered by federal tax credits and other incentives. Peterson said Volt owners would be eligible for the $7,500 federal tax credit provided on electric vehicles.
Several other automakers, including Chrysler Group, Nissan Motor Co., Ford Motor Co. and Toyota Motor Corp., are working on variations of plug-in hybrids or all-electric vehicles that are expected to reach the market over the next few years.