Wednesday, August 12, 2009

230 mpg Chevy Volt = Bad Math

Google software engineer Mark Chu-Carroll on the Chevy Volt's announced 230 mpg.

The Chevy Volt Gets 230 mpg? Only if you use bad math. : Good Math, Bad Math
Chevy has announced that for city driving, the Volt will get gas mileage of 230 miles per gallon.

That's nonsense. Pure, utter rubbish.

The trick is that they're playing with the definition of mileage. In city driving, the Volt is primary an electric car: it's powered by its batteries which you must recharge every night, not by gasoline. On average, you can drive it for about 40 miles on a full charge before it needs to start using any gasoline.

The 'mileage' figure, as it's presented, is really meaningless - because it's being presented for a situation in which the gasoline engine almost never runs at all.

They compute it by basically saying: 'If I fully charge the car battery every night, how far will I drive the car in typical city commuting conditions before it's consumed a gallon of gas'.

What if you drive your volt around the city all day? Your mileage will drop to around 50 miles per gallon once you've driven more than 40 miles. If you drive your car 100 miles in a day, you'll consume a bit over a gallon of gas. That's very impressive. But it's absolutely not what you'd expect after being told that it gets 230 miles per gallon.

The method that GM used to produce that mileage figure is extremely dishonest and completely uninformative. The 'real' effective mileage (excluding the cost of charging the car - which will be significant!) varies depending on the length of your commute.

My wife could commute in a Volt, and never put gas in it: her commute is about 12 miles each way - so she'd effectively have infinite mileage according to GMs method. If I commuted in a volt, I'd get something around 288 miles per gallon. (My commute is 24 miles each direction, leaving me with 8 miles per day running on gas; so about 6 days of my commute would consume a gallon of gas; that's 288 miles.) If one of my friends, who commutes 45 miles each direction per day, were to commute in a Volt, he'd end up burning a gallon of gas per day - getting around 90 miles per gallon.

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