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Hybrids like Toyota's Prius and plug-in vehicles such as the Chevrolet Volt run on systems other than purely gas-fired engines. The systems, or powertrains, that drive the so-called greener cars use electricity generated by batteries and electric motors to either supplement or largely take the place of internal-combustion engines. The Chevrolet Volt, billed as a rechargeable electric car, gets most of its power from an electric motor and a battery pack with a 40-mile range.
Is that the system used to rate every *except* the Volt is using an old standard, made for non-electrics, which is based on how much fuel is used (or an estimate of how much *would have been* had it not been electric). The Volt has a rating of 230mpg based on running the battery dry on a test track (to get the 40), then running an additional lap, which provides then with the "cost" in fuel for *after* the batteries run out.
Other systems do not use such a test, or use tests intended for pure gasoline based vehicles. As a result, there is no way to compare the Volt number with anyone else's, and get a correct result. Its likely that the new method used for the Volt will, however, be approved, and then we may see other vehicles using the same test system.
Sadly, the most likely *initial* result will be a bunch of companies jockeying for best "rating", by picking which ever method makes it "look" like they have the best result, even if the same vehicle, using the same test as the other, wouldn't look so good. This is going to require some watching, and is almost certainly going to generate a lot of FUD, false information, exaggerations, etc.