More electric cars means finding new standards to measure fuel efficiency

How to measure the fuel efficiency of the forthcoming Chevy Volt? It runs on electricity for the first 40 miles, then switches to gasoline. General Motors is saying the car has averaged 230 miles per gallon.
How to measure the fuel efficiency of the forthcoming Chevy Volt? It runs on electricity for the first 40 miles, then switches to gasoline. General Motors is saying the car has averaged 230 miles per gallon. (Jeff Kowalsky/bloomberg)
  Enlarge Photo    
By Peter Whoriskey
Washington Post Staff Writer
Tuesday, June 8, 2010

It used to be easy to know whether a car was a glutton for fuel. Federal standards defined miles per gallon and laid out precisely how the statistic should be measured.

But with the dawning of what enthusiasts hope is an era of electric cars, federal regulators are struggling to come up with a definition of auto efficiency for the next generation of technology. This year, two major auto manufacturers, General Motors and Nissan, will bring to market cars that plug in.

"There's no clear answer for how we should be informing the public about the efficiency of these new cars," said Mike Duoba, a research engineer at Argonne National Laboratory, outside Chicago. "The language we have been speaking -- mpg -- isn't sophisticated enough."

To fill the gap, researchers and Environmental Protection Agency officials have been conducting vehicle tests, researching driver habits and even running focus groups toward informing consumers about which cars are energy hogs.

Whatever metric they come up with is considered key to shaping consumer choices that in aggregate could profoundly affect smog and carbon emissions. The EPA is responsible for developing the fuel-economy labels posted on window stickers of all new cars and light trucks. A proposed rule is scheduled to be issued by August.

The onset of plug-in vehicles "will require new metrics to effectively convey information to consumers," according to an EPA statement. The new metrics are expected to change the way fuel-economy estimates are calculated and displayed.

But ask some of the people involved what exactly should be included in the new information for consumers, and you're likely to get sometimes bewildering answers.

Electric cars' efficiency might be measured, as some have suggested, in kilowatt-hours per 100 miles. But then how might a consumer compare that car's efficiency to a gasoline-powered car?

Maybe even more complicated: What if a car is powered by both electricity and gas? The forthcoming Chevrolet Volt, for example, will run on electric power -- it plugs in at home -- for the first 40 miles. Then it turns over to gas operation. How might its efficiency be measured?

With news releases, advertising and other fanfare last year, General Motors announced that under tentative federal standards the Volt achieved 230 miles per gallon.

Nissan, which is developing the all-electric Leaf for sales this year, shot back that its car achieved the equivalent of 367 mpg, citing an Energy Department conversion of electricity to gasoline.

But those numbers depended on complicated calculations that some have called into question. At the very least, they depend on making assumptions that may or may not predict a person's driving habits.

A person driving the Volt, for example, might in essence get astronomically high miles per gallon by never going farther than the car's 40-mile range. (Under 40 miles, no gasoline is used.)

So developing a standard for the Volt will probably mean assuming how much the car will be driven beyond the 40-mile battery range. Under calculations that Duoba and others have developed, regulators would probably assume that roughly 60 percent of miles traveled in a Volt will be on the battery, not gas.

But he remains skeptical that there can be any single way of describing the new cars that run on both energy sources. If he's right, the days of one simple statistic are gone.

"Everyone is saying we can boil this down to a single number," he said. "But with these new cars, we've added a dimension. Gasoline and electricity are in no way the same. You need a background in thermodynamics to appreciate what's going on."

© 2010 The Washington Post Company