How Do You Calculate MPG For Hybrid & Electric Vehicles?

How do you measure the efficiency of automobiles in terms of gasoline usage as automobiles increasingly rely on multiple fuel sources, or on electricity alone. How do you give consumers accurate information about the financial and environmental costs of driving?

General Motors (GM) says its Chevrolet Volt will get 230 mpg fuel economy in city driving. But the Environmental Protection Agency (EPA) is still finalizing rules for how it will measure fuel economy on the Volt or other cars that can be plugged into an electrical outlet, so manufacturers' claims are not official. The 230-mpg figure for the Volt, which will be able to run on both electricity and gasoline, doesn't provide energy equivalent information for the use of electricity. The Volt's mpg claim is also based only on city driving -- a standard that favors electric cars.

According to GM, the Volt's battery, when fully charged, can power trips of about 40 miles. 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.

According to Nissan, its all-electric vehicle, the Leaf, will get 367 mpg. That number, a combined city/highway figure, is based entirely on converting electricity usage into a petroleum equivalent, because the Leaf won't use gasoline at all. Although the mileage number is lofty, it doesn't mean that operating the Leaf will be seven times as efficient as driving a 50-mpg Toyota Prius hybrid, because electricity costs vary by region and even by time of day. It also is more difficult for consumers to calculate their savings, because electricity costs aren't posted at roadside stations like those for gasoline.

Researchers are struggling to determine the best way to calculate a single fuel-efficiency rating for vehicles that rely on both electricity and gasoline. Should the conversion factor be based on how much energy each fuel source produces? On how much petroleum is required? Or on the level of harmful emissions produced?

The Department of Energy has come up with guidelines that draw from several competing approaches. Citing those guidelines, Nissan says 82 kilowatt hours of electricity are the equivalent of one gallon of gasoline for the all-electric Leaf. Using the same standard, the GM Volt's city fuel economy could drop to about 130 mpg, if the car's expected electricity consumption were factored in. (WSJ, 8/26/09, Graphic Courtesy WSJ)