Long the Barometer Used to Gauge Fuel Efficiency in the U.S., the Gasoline-Based Measure Falls Short for Electric, Hybrid Cars
By CARL BIALIK
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 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.
One problem is that in hybrid vehicles, 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.
Since the Volt's economy varies so much depending upon the distance between charges, GM had to make numerous assumptions about how Volt drivers would use their car before the company came up with the 230-mpg figure. By studying government surveys on driving patterns, the company predicted a range of fuel-economy numbers that Volt drivers would see, and aggregated those into one figure.
The EPA, which does its own evaluations of automobiles' fuel efficiency, could arrive at a different number. "We took that into account when we chose 230 as the number we went with," Mr. Lauckner says, calling it conservative. "We say 'at least 230.'" The company, however, hasn't disclosed the equivalent figure for highway driving, saying only that the car's combined city-highway rating should be in the triple digits.
Nissan, meanwhile, says its all-electric vehicle, dubbed 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 -- at least, not yet.
Another problem with applying the mpg standard to next-generation automobiles is that car makers are accounting for electricity usage differently. In the case of GM's Volt, electricity usage isn't incorporated into the 230-mpg figure at all. GM says that since the EPA is likely to separate fuel consumption from electricity usage on labels for cars such as the Volt, it decided not to factor in electricity use.
That, too, could pose a challenge for car buyers. Consumers are accustomed to figuring their fuel costs from mpg figures, but running the Volt will have additional costs.
The complications that mpg numbers could pose for consumers as more electric cars arrive on the market should lend fuel to a related movement to jettison mpg in favor of a more consumer-friendly barometer. Last year, Richard P. Larrick, who teaches management at Duke University's Fuqua School of Business, co-wrote a paper in Science suggesting that gallons per mile -- a measure whose metric equivalent is standard in much of the world -- would be far more useful for conveying fuel consumption and potential savings.
Flipping mpg on its head shows that the biggest fuel savings come from upgrading from the most fuel-inefficient vehicles. For instance, a plug-in car with a 200 mpg label might sound a lot more impressive than a 50-mpg hybrid. But over 500 miles of driving, the plug-in saves just 7.5 gallons compared with the hybrid.
Gallons per mile has gained some traction. Prof. Larrick says that some major auto reviews now include the figure. But just as the metric system remains far from U.S. adoption, mpg appears here to stay for a while.
Meanwhile, 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?
Jeff Gonder, a research engineer at the Department of Energy's National Renewable Energy Laboratory who serves on a panel advising the EPA on these issues, says there might not be one ideal approach. "If it's for consumers, maybe a cost comparison is best," Mr. Gonder says. "If it's for studying global warming, maybe it should be on the basis of greenhouse gases used."
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.
Mr. Gonder says the committee is leaning toward recommending that the numbers for gasoline consumption and electricity usage be reported separately, as GM is anticipating. "No matter what you do, I think it will be a challenge with these vehicles to convey the information people really want."
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Write to Carl Bialik at numbersguy@wsj.com
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