Miles per gallon? Gallons per mile is another view

WASHINGTON — With the gas well past $4 a gallon, everybody wants to save money. But calculating miles per gallon can be misleading.

Strange as it may sound, rating cars at gallons per mile may be more useful, say a pair of university researchers.

Richard Larrick and Jack Soll got to discussing fuel efficiency while carpooling to work at Duke University's Fuqua School of Business. The professors study how people perceive things and decided to look into mileage ratings and what they tell consumers.

The result is a paper called "The MPG Illusion," which appeared last week in the journal Science.

In essence, they say, don't turn your nose up at what may seem like a small gain. It can still mean big savings.

Not everyone is a good candidate for a tiny car, Larrick explained. A family of five or six needs a larger vehicle. But moving to even a slightly more efficient large car can be a big savings.

"We realized improving low mpgs is where the big bang is," Larrick said. "But we realized that people were not going to understand that."

He stressed that they are not advocating buying inefficient cars, but rather pointing out that the most inefficient are the ones that need to be replaced, even if the extra miles per gallon seem small.

"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," said Soll.

Jack Gillis of the Consumer Federation of America called their paper "extraordinarily profound in its simplicity."

The report shows that people with inefficient cars can save substantially by just moderately increasing their fuel efficiency.

"I am convinced that the average, extraordinarily frustrated owner of a fuel-inefficient car has no idea that making a small improvement will save more money and will save the environment" more than a larger improvement from a more efficient car, Gillis said.

How to do the math

So why does it help to look at gallons per mile instead?

Well, that tells you how much gasoline is used or saved over a given distance, say a year's driving of 10,000 miles.

Gillis calculated that at $4 a gallon, over 10,000 miles, an improvement from 12 mpg to 13 mpg would save $256.

But surprise: For the owner of a 33 mpg car to save that much money, he'd have to trade for a car that got 40 mpg.

Another example:

A couple drive a 25 mpg sedan. They trade it for a 50 mpg hybrid.

A family with mom, dad and three kids has a 10 mpg SUV. They trade it for a 20 mpg station wagon.

Sounds like the couple did better, at least in mpg.

But lets look at gpm.

At 25 mpg, the couple burned 400 gallons over 10,000 miles, and their new 50 mpg hybrid cuts that to 200. They save 200 gallons.

At 10 mpg, the family's SUV burns 1,000 gallons over 10,000 miles. At 20 mpg the station wagon burns 500 gallons. They save 500 gallons.

Would it be better for everybody to switch to the most efficient car? Sure, but not every family will fit in it.