Here's a quick bit of obnoxious bad math. I saw this myself in a link to an AP article via Salon.com, and a reader sent me a link
to the same story via CNN. It's yet another example of what I call a metric error: that is, the use of a measurement in a way that makes it appear to mean something very different than what it really means.
Here's the story. Chevy is coming out with a very cool new car, the Volt. It's
a hybrid with massive batteries. It plugs in to your household electricity when you're home to charge its batteries. It operates as an electric car until its batteries start to get low, and then it starts running a small gas motor to power a generator. It's a very cool idea. I'm honestly excited about cars like the volt - and Google helped develop the technology behind it, which biases me even more in its favor. So you'd expect me to be very supportive of the hype around it, right? I wish I could. But GM has decided that the best way to promote it is to use bad math to tell lies to make it look even better than it really is.
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.
Plug-in hybrids are a new class of car. You can't
really describe their efficiency compared to a conventional gasoline-powered car using a single familiar figure. You could
present energy efficiency in terms of a unit like
"distance per kilojoule", but most people won't have a clue
of what that means. The honest way to describe it is to say "Up to 40 miles without consuming gas, and then 50 miles per gallon". That's not
so horribly difficult, now is it?
But it doesn't sound nearly as impressive as "230 miles per gallon".