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Miles per gallon an accurate measure of new hybrids' efficiency?

November 15, 2010

General Motors promises its forthcoming Volt hybrid electric car will push fuel-economy levels to new heights. It also could spark an overhaul of the miles-per-gallon standard, a number that doesn’t tell consumers enough about the next generation of vehicles.

In recent weeks, GM has touted the Chevrolet Volt’s expected 230 mpg fuel economy in city driving. The big number dwarfs the mileage of any car on dealer lots.

Yet high mileage claims for the Volt and other planned plug-in automobiles highlight what the Wall Street Journal contends is a deep flaw with the mpg standard: As automobiles increasingly rely on multiple fuel sources, or on electricity alone, gauging their efficiency in terms of gasoline risks giving consumers inaccurate information about the financial and environmental costs of driving.

Reporter Carl Bialik wrote Aug. 26 that, in hybrid vehicles, one problem is that mileage variation could be extreme, depending on which fuel source is being used.

In addition, the Environmental Protection Agency hasn’t finalized rules for how it will measure fuel economy on the Volt or other cars that can be plugged into an electrical outlet. Until then, manufacturers’ claims won’t be fully comparable.

For instance, the 230-mpg figure for the Volt, which will be able to run on both electricity and gasoline, doesn’t incorporate the use of electricity. The Volt’s mpg claim also is based only on city driving—a standard that favors electric cars.

Different drivers "will get wildly different numbers for the exact same vehicles," since some drivers will rely more on gasoline, says Jon Lauckner, GM’s vice president of global product planning. "That’s why the EPA needs to develop a robust methodology."

Calculating the Volt’s fuel economy is complicated because of its two power sources. The Volt’s battery, when fully charged, can power trips of about 40 miles, according to GM.

Battery-fueled trips won’t use any gasoline, although they will require electricity. Once the battery runs out, the engine begins drawing on gasoline.

So drivers who use the Volt only for short trips, relying only on electricity, in theory could enjoy infinite fuel economy. Meanwhile, drivers who routinely use the Volt for long journeys, where gasoline power would be necessary, would see a far lower fuel economy than the 230 mpg advertised.

GM’s Mr. Lauckner says the company based its numbers on EPA draft regulations. An EPA spokeswoman says the agency doesn’t have a draft available for public consumption.