4 Jun 2009 7:44 am   //   Filed under: Labeling, Planet earth, Stray data

Math is important

I just read an interview in Good magazine with Richard Larrick, a Duke business professor who advocates changing the “miles per gallon” standard we use to rate car efficiency. The problem? Basically, mpg statistics mislead our brains.

Larrick and professor Jack Soll have been on a crusade to adopt a “gallons per mile” standard. What’s the difference? Here’s a story about their work from 2008. It says:

Most people ranked an improvement from 34 to 50 mpg as saving more gas over 10,000 miles than an improvement from 18 to 28 mpg, even though the latter saves twice as much gas. (Going from 34 to 50 mpg saves 94 gallons; but from 18 to 28 mpg saves 198 gallons).

“These mistaken impressions were corrected, however, when participants were presented with fuel efficiency expressed in gallons used per 100 miles rather than mpg. Viewed this way, 18 mpg becomes 5.5 gallons per 100 miles, and 28 mpg is 3.6 gallons per 100 miles — an $8 difference today.

I had never thought about this before. But it makes sense: The higher the mpg number, the smaller the significance of each mile, because you cover more distance before you need to tap that extra fuel. We are used to thinking each number in a rating scale has the same value. It’s misleading.

The professors are using their math to defend small improvements in low-mileage vehicles—a strong argument for hybrid SUVs, which are scoffed at by most environmentally minded people. In fact, it makes a big difference. Here’s Professor Soll’s argument:

“There are significant savings to be had by improving efficiency by even two or three miles per gallon on inefficient cars, but because we communicate in miles per gallon, that savings is not immediately evident to consumers.”

I’m convinced.