It’s an eye-popping number: 230 miles per gallon. General Motors says that’s the mileage its new plug-in car, the Chevrolet Volt, will get in city driving, assuming it goes on sale as planned in late 2010.

It’s also misleading. The Volt will get astounding “gas” mileage because it won’t exactly be powered by gas. At least a lot of the time it won’t, if drivers use the car the way GM envisions. That’s the whole point of an electric car: To propel a vehicle with something other than petroleum. If the Volt were powered mostly by a windmill or a nuclear reactor, it would also get great gas mileage, since the fuel would be coming from some other source.

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