Fans of the Chevrolet Volt have followed its development closely, but few even of the faithful expected to see numbers as big as the ones coming out of General Motors today. The tentative EPA methodology works out to a city efficiency rating of 230 mpg.

Talk of mile-per-gallon equivalents can get confusing quickly, but the EPA's rating system relies on a system that's intended to put cars on an equal playing field. Whether it successfully does that, especially in relation to extended-range electric vehicles like the Chevrolet Volt, is a question that's not yet fully settled, but it's the most independent, authoritative view on the matter to date.

The methodology used for the testing itself still isn't finalized, but it is far enough along that GM feels comfortable releasing the city efficiency figures. Overall, or composite, fuel efficiency isn't being released, but it's expected to top 100 mpg.

"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," said GM Chief Executive Officer Fritz Henderson. "EPA labels are a yardstick for customers to compare the fuel efficiency of vehicles. So, a vehicle like the Volt that achieves a composite triple-digit fuel economy is a game-changer."

The EPA methods being developed use a kilowatt-hours per 100 miles traveled metric to define the electric-only efficiency of plug-in hybrids and extended range EVs like the Volt.

The figures released today represent pre-production vehicles in preliminary testing. Full production vehicle testing with a presumably final EPA methodology won't take place until next year, closer to the Volt's late 2010 launch.

For a  thorough run-down of the issue and the possible implications of assumptions made about how drivers will use the car's electric-only range, read John Voelcker's post on the Volt's 230 mpg  EPA rating over at Green Car Reports.

[GM]