Here’s an interesting suggestion for saving gasoline, your bank account and the environment: Forget about how many miles per gallon your car gets.

A couple of professors at Duke University have been promoting the idea this month after determining that the math-challenged American public is fairly confused about fuel efficiency because of our fixation with mpg.

And it’s not as crazy as it sounds.

Consider this example: Smith trades in a 10 mpg SUV for a slightly less inefficient 15 mpg SUV. Jones, meanwhile, almost triples his fuel efficiency by trading in a 20 mpg minivan for a 55 mpg hybrid.

Who is going to cut their gas usage more? Smith, the SUV driver.

In fact, assuming they both drive 15,000 miles a year, Smith will save $2,000 worth of gas and Jones will save $1,908. Now I, too, had understood there would be no math in this column. But times are pretty desperate with gas now rising toward $4.10 a gallon, and here is a case where a little math could change the way you look at the vehicles in your driveway and how you approach your next purchase.

The business professors at Duke started talking about fuel economy and mpg while carpooling to work in a hybrid. Then they asked college students trick questions like the one above, only not so obvious.

For example, which would save more gas: a) upgrading from 16 to 20 mpg, or b) upgrading from 34 to 50? The correct answer is a, although the college students tended to say b.

That led to an article in the journal Science, a call for a national math lesson and a lot of head scratching. “It’s been nice to get the word out,” said Jack Soll, one of the professors.

What’s basically happening here, mathematically speaking, is a case of diminishing returns.

Even a small gain in mpg for a gas-guzzler can have a huge impact on fuel consumption simply because it’s burning so much more to start with. Getting 5 mpg more out of a Ford F-150 could save $1,500 a year.

More fuel efficient vehicles, on the other hand, are already using less, so cutting their consumption by the same percentage carries a much smaller savings. Getting 5 mpg more out of a Honda Civic could save $200 a year.

Of course, trading in the F-150 for the Civic is the best deal all around, excluding cargo space, of course. The professors took pains to say that improving fuel efficiency is a good thing no matter what you’re driving.

But, if you’re looking to compare fuel efficiencies and the cost of driving one car against another, they said, forget mpg.

A much better way, they said, is to figure out “gpm” – or how many gallons it takes to drive your car 10,000 miles. Using gpm, you see that improving from 10 to 15 mpg will save 333 gallons of gas every 10,000 miles, while going from 30 to 50 will save 133 gallons.

For a more complete explanation and chart, see this Science Daily article (http://www.sciencedaily.com/releases/2008/06/080619142118.htm).

The Duke research isn’t likely to change American car ratings from mpg to gpm any time soon. But, given the cost of a tank of gas, a little math won’t hurt us.

E-mail this entry to a friend