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What it really costs to fill up a plug-in car EPA label gives two different fuel economy figures for the Chevrolet Volt. The challenge of figuring out what your real fuel economy might be is largely up to you. By Peter Valdes-Dapena, senior writer

NEW YORK ( — Even if they aren’t always terribly accurate, EPA fuel economy estimates have at least made it easy to compare the gas mileage of one car to another. But now that plug-in cars are entering the market, things are about to get much more complicated.

How do you compare the fuel efficiency of a car that runs on gasoline to one that plugs into an outlet?

Last week, Nissan and General Motors each released preliminary EPA fuel economy labels for their plug-in models. The labels, particularly for the Chevrolet Volt, which runs on both plug-in electricity and gasoline, seem sure to provide at least as much confusion as assistance to potential car buyers.

We’ll try to sort out some of the trickier issues.

Q: The EPA’s label seems to say that the Nissan Leaf, which uses no gas, gets 99 miles per gallon. How is that possible?

A: Actually, the Nissan Leaf gets 99 miles per “gallon equivalent,” or MPGe. That means the amount of electricity the Leaf uses to drive 99 miles is equivalent to the amount of energy in a gallon of gasoline. (If you really want to know, there are 33.7 kilowatt-hours in a gallon of gasoline.)

The Chevrolet Volt, by comparison, gets 93 MPGe when using electricity from its battery.

Q: If it’s not burning gasoline, why do I care about MPGe?

A: You probably don’t. The concept of MPGe was created as a way to compare the efficiency of an electric car to that of a gasoline-powered car which, the Environmental Protection Agency says, consumers indicated was important to them.

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