Is the GM Volt mileage claim legit?

General Motors says its electric car will get 230 miles per gallon in the city. But there are still questions about how plug-in vehicles should be rated on fuel efficiency.

Martin LaMonica Former Staff writer, CNET News
Martin LaMonica is a senior writer covering green tech and cutting-edge technologies. He joined CNET in 2002 to cover enterprise IT and Web development and was previously executive editor of IT publication InfoWorld.
Martin LaMonica
4 min read

WARREN, Mich.--General Motors' announcement on Tuesday that it expects that the Chevy Volt will get an eye-popping 230 miles per gallon begs an obvious question: how can the mileage of electric vehicles be compared to gasoline cars?

It's a problem that the Environmental Protection Agency is working on with the Department of Energy, the Society of Auto Engineers, and California, an EPA representative said on Wednesday. But that system for testing mileage is still in development and not yet public.

The EPA also put out a statement on Tuesday saying that it has not tested the Volt for mileage yet and "cannot confirm the economy values claimed by GM." GM said that its mileage estimate, including triple digit combined city and highway driving, was based on a draft methodology developed by the EPA.

The lack of verifiable tests, however, hasn't stopped automakers from tantalizing consumers. The all-electric Nissan Leaf, due in late 2010, boasts the equivalent of 367 miles per gallon, and the electric Tesla Roadster claimed over 100 miles per gallon mileage as well.

Pressed on how mileage numbers for the Volt were arrived at, GM executives offered some details, saying that the number will vary depending on how far people drive before they replenish the car's batteries.

"I'm confident that we will be in triple digits" with Chevy Volt mileage, said GM CEO Fritz Henderson at a press conference on Tuesday. Martin LaMonica/CNET

The draft EPA methodology figures that a plug-in electric vehicle driver will go a certain number of miles on batteries alone and then another portion on the gasoline engine, explained Frank Weber, the global vehicle line executive for the Chevy Volt. To arrive at the mix between battery versus gasoline, the EPA is studying average American driving patterns, executives said.

The EPA is also developing another, less familiar metric for electric vehicles. In the Volt's case, it will take 25 kilowatt-hours to go 100 miles. Weber said the models behind the EPA methodology are "robust," adding that he expects the EPA to disclose more about the tests later this year.

To come up with 230 miles per gallon for city driving, GM assumes that Volt owners charge the car's batteries once a day, which enables them to do the majority of their driving from electricity drawn from the socket. The Volt, due late next year, is designed to run 40 miles on electric charge and then use a gasoline engine to sustain the battery for longer trips.

Triple digit combined fuel efficiency is certainly impressive--the Toyota Prius and Honda Insight hybrids both sport combined mileage of about 50 miles per gallon depending on driving style.

But immediately after GM's announcement, people began complaining that the claim is misleading.

For example, comparing the Volt to the Prius with that methodology is not useful, argued Darryl Siry, the former chief marketing officer at Tesla Motors and now a consultant with green-tech companies. By the EPA measure, it would appear that the Volt is many times more efficient than the Prius. But the 230 miles per gallon rating more accurately reflects how much gasoline has been consumed rather than the overall efficiency of the system, he said. "People will improperly conclude that the Volt is about five times more efficient that the Prius, which simply isn't true," Siry wrote.

A common way of doing comparisons is converting the embedded energy in gasoline to batteries, which a 2000 Department of Energy rule does in addition to considering the efficiency of the overall energy delivery system.

In the case of the Volt's city mileage, fuel economy will begin to drop off when drivers go beyond 40 miles before recharging. The Volt's electric driving range was chosen specifically because U.S. Department of Transportation research shows that almost 80 percent of Americans drive under 40 miles a day.

In the EPA model GM has followed, those first 40 miles equate to "infinite mileage," since it was charged from the grid and no gasoline was burned. But to consider electricity as infinite fuel efficiency can be misleading given that some energy--be it coal, natural gas, or nuclear--went into the delivery of electricity to charge the batteries.

Observers said advertising the Volt's fuel economy allows GM to bring attention to its products and tout efforts to improve the efficiency of its overall fleet, with one analyst calling triple digit mileage "bragging rights."

Asked whether he thought GM was overpromising by advertising the 230 mileage number, GM CEO Fritz Henderson said no. "We are quite confident under EPA testing methodology, that's what what the customer will see in the city," he said during a press conference, adding that GM is testing the Volt in both hot and cold climates to gauge the sensitivity of the battery system to temperature.

But when it comes down to choosing a plug-in electric vehicle or not, customers need to consider if they fit into the "sweet spot" for plug-in electric vehicles, which is about 20 to 60 miles a day, said Larry Nitz, the executive director of hybrid and electric power train engineering at GM.

"Plug-in vehicles are not for everyone. If you drive long long distances or you can't readily get a charge, it's probably not for you. But if you can get to a plug two times a day, this is a huge huge victory," he said. "It is a far more complicated way of articulating to the customer how much your fuel economy is."