General Motors today announced that its Chevrolet Volt electric vehicle, set to begin production in late 2010, is expected to achieve city fuel economy of at least 230 miles per gallon, based on a new U.S. Environmental Protection Agency (EPA) formula that neither GM nor the EPA has defined very well publicly.

For comparison, Toyota's Prius currently is rated to get up to 51 mpg during city driving.

Here's what we do know: The Volt will have two modes of operation. In "electric" mode, the Volt will not use gas or produce tailpipe emissions because the car will be powered by electrical energy stored in its 16-kilowatt-hour lithium ion battery pack. When the battery charge gets too low, the Volt is designed to automatically switch to "extended-range" mode and use a gas-powered engine-generator to produce electricity to power the vehicle. The energy stored in the battery supplements the engine-generator when additional power is needed during heavy accelerations or on steep inclines.

The EPA's plug-in electric vehicle federal fuel economy methodology assumes plug-in electric vehicles will travel more city miles than highway miles solely on electricity, GM said in a statement. (At low speeds and short distances, the Volt is expected to operate solely on battery power, without touching the reserves in the gas tank.)

Still, the Volt's actual gas-free mileage will vary depending on a number of factors, including how far the car travels, the weight of the cargo and passengers, and whether the air conditioning is used. Based on the results of unofficial development testing of pre-production prototypes, GM reports the Volt can achieve 40 miles of electric-only, petroleum-free driving in both EPA city and highway test cycles, before needing a recharge (which can be achieved at a household outlet).

During a live blogging event today on GM's FastLane Blog, Jon Lauckner, GM's vice president for global program management, reinforced that the 230 mpg rating is a preliminary estimate "based on Volt development testing with our pre-production vehicles and the draft federal fuel economy methodology developed by EPA for EREV [Extended Range Electric Vehicle] vehicles like the Volt."

GM's reported gas mileage at this point may "end up being very misleading because [customers] won't get that mileage in the real world," Brett Smith, an alternative technology analyst at the Center for Automotive Research in Ann Arbor, Mich., told Bloomberg.