General Motors made some big headlines this morning with word that its Chevrolet Volt electric car will get 230 miles per gallon in the city. That’s an impressive feat to be sure. It will certainly leap frog any car on the market except for all-electric cars like the Nissan Leaf or the Tesla Roadster when the Volt goes on sale late next year. But the Volt has an advantage for some drivers because its gasoline engine keeps recharging the battery, so drivers can drive far longer than the Leaf’s 100 miles. In auto industry parlance, the Volt is an extended-range electric vehicle.

But there is one potential quandary for the government, GM, and any other producer of EVs. The fuel economy and range ratings can vary greatly depending on how people drive them, and far more so than the difference you get by driving conventional gasoline-powered cars with a lead foot. EVs recharge the battery when braking, so the range (and in Volt’s case mileage) will be much better in the city. On the highway, drivers will use more electricity and charge less. An electric car can get half its range in pure highway driving than it might in the city.

So consumers may start griping if they don’t get the published range or fuel economy numbers. They’ll complain not only to the manufacturer but to the government. So right now the feds are studying hard to figure out how people will drive these cars and what rating they should put on them. GM insiders say the Volt could easily get less than 100 mpg on the highway, maybe even as low 50 mpg if it’s driven hard. But they’re thinking that the car will get a combined rating for city and highway mileage of 124 mpg. That’s still mighty impressive. This issue isn’t about technology. It will be about how the government decides to rate the cars and how the manufacturers will communicate the message. Consumers, too, will have to be more open-minded and realistic about their expectations.

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