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April 1, 2011

News & Features

MPG + e: Mileage measurements can be puzzling to electric-car shoppers

New York Times News Service

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It's not easy comparing the mileage of electric cars with others in their class. (Thinkstock)

According to the government, the car with the highest mileage per gallon on the market doesn't use a single drop of gasoline.

The 2011 Nissan Leaf, which was delivered to its first owners in December, runs entirely on battery power. But the Environmental Protection Agency (EPA) says it can travel 99 miles on the equivalent of a gallon of fuel.

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The 2011 Nissan Leaf runs entirely on battery power. Its sales sticker says it can reach up to 106 MPGe in the city. (Nissan)

Confused? You're not alone. The mileage-equivalent ratings, meant to help potential buyers compare electric cars with others in their class, are befuddling to some consumers who see them as an automotive example of comparing apples and oranges.

"It's a whole new world that needs to be rated," Nissan spokeswoman Jeannine Ginivan says. "It is, for sure, complicated, since there is really no gallon. For now, the consumer is going to have to decipher everything and see how to make it work for them."

Traditionally, the fuel-economy rating for vehicles with internal-combustion engines is calculated from emissions generated during a series of tests.

Things got hairy with the Leaf. The EPA worked out a formula in which an electric car using 33.7 kilowatt-hours of electricity was considered equivalent to a standard vehicle using a gallon of gasoline. Because there wasn't any physical fuel, the rating was renamed miles-per-gallon equivalent, or MPGe.

The electric vehicle will be sold with a sticker saying that it can reach up to 106 MPGe in the city. The previous record holder, the Toyota Prius hybrid, was rated at 51 mpg in urban settings.

More to sort out
  • Miles-per-gallon testing isn't the only category that's causing confusion. The distance an electric vehicle can go on a single charge also varies, depending on whom you ask. The Leaf, for example, was found by the EPA to go 73 miles on a single charge. The Federal Trade Commission pegs the distance at up to 110 miles. Nissan, which used a slightly different testing system, says the car can travel 100 miles on one charge.

The scores get even more complicated with the 2011 Chevrolet Volt, a hybrid plug-in that switches to gasoline when its electricity supply runs out.

In addition to an umbrella figure of 60 MPGe, the government gave the Volt a 93-MPGe rating for when it is driven only in its electric mode and a 37-mpg figure for when it uses gasoline only. But few, if any, Volt drivers will stick to just one mode. They are more likely to use gas and electricity in varying ratios.

The EPA figures also include expected tailpipe emissions. Since it doesn't need a tailpipe, the Leaf will release no carbon dioxide, according to the agency.

Some environmentalists aren't pleased with the prognosis and suggest using what's known as a well-to-wheel analysis instead. The process would consider all the greenhouse gases released from the time the electricity is first generated until it is sent through transmission lines to charging units.

Based on such measurements, the Leaf would rack up more than 250 grams of carbon dioxide and other emissions every mile, according to data from the Energy Department's Argonne National Lab. Gasoline-fueled cars, on average, release 450 grams a mile.

"It doesn't matter where greenhouse-gas emissions are emitted, because they'll still affect climate change around the world," clean-energy consultant Sandy Thomas says. "The fact that the emissions came from a coal plant producing electricity in Utah is just as bad as if they came out of the tailpipe."

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