Questions Surround GM's 230 mpg Chevy Volt Claim

Posted: Aug 12, 2009 11:17 a.m.

General Motors jolted the automotive world yesterday, announcing that its upcoming electric-drive Chevy Volt will earn an EPA city mileage rating of 230 mpg. Given a day to digest the news, the automotive press smells something fishy - and even the federal government says it can't explain GM's math.

The Volt is one of several so-called Extended-Range Electric Vehicles, or EREVs, in development. An EREV functions as an electric car until its batteries are depleted to a certain level, then starts a small gasoline engine, which acts as a generator to recharge the batteries. The Volt's battery-only range, according to GM, is about 40 miles.

Most Americans drive less than 40 miles per day, meaning that many Volt drivers could get through an average day without using any gas at all. That possibility, however, makes measuring the car's predicted fuel economy a tricky process.

Motor Trend notes that GM's announcement said the Volt "will achieve ‘at least' 230 mpg under the ‘tentative' EPA testing process for EREV vehicles." Under the draft guidelines, Motor Trend reports, "The EPA will put more emphasis on city mileage when testing EREV vehicles." The proposed measurement system rates vehicles in kilowatt hours per 100 miles, then converts that measurement to its equivalent in miles per gallon. "Under these rules, GM expects the Volt to consume as little as 25 kilowatt hours per 100 miles, which works out to 230 mpg when both the battery and gasoline tank are empty."

Questioning the calculation, Nissan immediately poked fun at the GM announcement, posting an announcement on its NissanEVs twitter account noting that the upcoming 2010 Nissan Leaf would be rating for 367 mpg under the same formula - even though the Leaf, a pure electric car, doesn't have a gas tank.

The EPA distanced itself from the announcement, telling GreenCarAdvisor "EPA has not tested a Chevy Volt and therefore cannot confirm the fuel economy values claimed by GM," though they added, "EPA does applaud GM's commitment to designing and building the car of the future."

So what kind of mileage can a Volt driver expect? GM claims the Volt has a 300 mile range after the gasoline engine ignites, but the company hasn't released official numbers telling the press the size of the Volt's gas tank. Kicking Tires notes, "If it packs a 10-gallon gas tank...our simple math finds that it will get 34 mpg overall. GM says it will get 40 mpg once the battery is depleted, which would mean an exceptionally small 8.5-gallon gas tank."

Your mileage, then, will vary greatly based on how far you drive. For 40 miles, the Volt uses no gas. After that, it's apparently about 40 mpg. So if you commute 40 miles or less per day, you could expect an infinite number of miles per gallon. For every mile you drive over 40, the number drops precipitously, stabilizing at around 40 mpg as the needle drifts toward "E."

But that's a mouthful, and not nearly as easy to market as 230 mpg.

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