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When is 100 MPG not 100 MPG?

Posted by: David Welch on September 26, 2008

On this blog, my colleague David Kiley writes that General Motors could get a fuel economy rating as high as 100 miles per gallon for the Chevrolet Volt when the car goes on sale in 2010. This was reported by another news agency early today. That would be quite a boast to have on billboards and magazine ads. It will also be a great technological achievement.

But hang on a second. First, GM has not reached an agreement with the Environmental Protection Agency on the Volt’s rating. The new administration, whether its Barack Obama or John McCain who takes the White House, will have oversight of the EPA. No doubt, they will weigh in on it. Second, and more importantly, neither GM nor the EPA has even settled yet on how to rate such a car. The Volt will drive only on its electric battery for 40 miles before a gasoline engine kicks in to charge the battery. So how do you rate it? A driver who goes less than 40 miles and almost never uses gasoline gets infinite fuel economy. One who goes well beyond the 40 electric miles gets worse fuel economy. One company engineer told me that GM and the EPA have been throwing different models back and forth that seek to profile the average driver. If they can agree to a driving pattern, they could come up with an estimated fuel economy rating.

That brings up a thorny issue for GM. If a proud owner of a new Volt drives, say, 41 miles, then he will use very little gasoline. Add the 40 gas-free miles to the one mile he drove with the gasoline engine running and his fuel economy performance will be off the charts. He goes home, recharges the battery and does it all again the next day. He’s over the moon with his car’s performance. As an aside, I picked 41 miles because that the daily average miles driven for American motorists.

But if another Volt owner ends up on the road all day with the engine running and driving until the tank is empty, then mileage performance for that tank of gas drops mightily. It might go below 100 mpg or whatever cosmic rating the EPA might give the Volt. At this point, GM engineers working on the car don’t know what the rating will be or what its real-world mileage performance will finally be. When the company initially talked about the Volt concept, they said it would get a minimum of 50 mpg if the driver used the entire tank of gasoline. Internally, Vice Chairman Bob Lutz has said there are “horses for courses.” If someone drives 100 miles to and from work and takes a road trip to the cabin every weekend, the Volt may not be for them.

That kind of driver would still get great fuel economy. But GM has to manage expectations if the final rating grated by the Environmental Protection Agency is in triple digits. If the Volt performs beneath its rated mileage after using a full tank of gasoline, the owners need to know why. This is a brave new world for cars. GM will have to make sure its buyers understand what they’re getting. Otherwise, that 100 mpg (or whatever the final rating ends up being) might not be 100 mpg.

Reader Comments


September 28, 2008 1:06 AM

Welch's mistake is in his assumption that Kiley knows what he is talking about. Is this a case of the blind leading the ignorant or two BW writers imitating Mo and Larry doing slap-shtick? Calculating the average mpg for a hybrid vehicle is fairly straight forward. Applying stoichiometry to the oxidation-reduction reaction of one mole of Octane we can calculate the energy in Joules. For one gallon of Octane, we merely scale up by linear proportions. If we run the car on only gasoline, we can graph the average mpg at various range of driving. Next, we can calculate the Joules consumed by the electric motors while car is running on pure electric mode assuming the battery is depleted at 15% reserve. To compare apples with apples, we convert the Joules produced from battery(assuming a 75% efficiency) to equivalent gallon of gasoline. See above ratio of Joules to one gallon of gallon Octane. Divide the miles driven by the calculated equivalent gallon of gasoline will give you the average mpg. The combined mpg can be discounted by a coefficients due to road friction, wind drag,etc to arrive at an average hybrid mpg.


September 30, 2008 2:59 PM

Heaven forbid someone drive the Volt (can this actually be done?), on whatever the same test is for all ICE cars and see how much gas it uses. EPA isn't currently doing seperate test for 'people who only drive 40 miles a day in the city' so why do it for the Volt. Give it the exact same test at the same speeds and distances and let the cards fall where the will. If someone drives 100 miles to work and to the cabin on the weekend (with 4 people) a SMART car isn't for them but the EPA fuel # stays the same, why is the Volt any different?

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