U.S. regulators are working on a new formula for measuring the Volt's mileage. GM says it doesn't know what that new number will be.

One of the problems in automotive journalism is that we have to report the performance claims provided by automakers, even when the opportunity to have those claims independently checked is not available. Or when we know they are laughably unrealistic.

So, chalk this one down in the category of "Yeah, right...".

Chevrolet says its initial mileage rating claim for the Chevy Volt of 230 miles per gallon is a no-go. They say the U.S. federal government is currently working on a new formula for calculating the Volt's mileage.

The complication comes from the fact that the Volt is a range-extended electric car that gets only 40 miles (65 km) on a full charge but also features a gasoline engine that recharges the battery. Finding a formula to get a proper mileage rating from that is not simple, since fuel-economy becomes dependent on how full the battery is, which varies as the vehicle is driven. That job is up to the EPA (Environmental Protection Agency), which has not yet decided on one.

"We just don't know right now," said Micky Bly, GM's director of global electrical systems, regarding what the new number will be.

The Society of Automotive Engineers (SAE), a group that sets standards for the automotive industry for measuring mileage, horsepower and other ratings, is working on energy-use comparative standards for electric and plug-in cars. It has recently developed a standard for rating plug-in hybrids which it expects the EPA to adopt.

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