This is really interesting. The authors are advocating measuring "gallons per mile" rather than "miles per gallon", pointing out that a change from 12 mpg to 13 mpg can save more than a change from, say, 30 mpg to 35 mpg. I never did the math on it this way before, but of course once you calculate it, it's very true. Here's an excerpt from the CNN article.

"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. For the owner of a 33 mpg car to save that much, mileage would have to go up to 40 mpg, he said.

Here's how it works.

A couple drives a 25 mpg sedan. They trade it for a 50 mpg hybrid, a 25 mpg improvement.

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

Sounds like the couple did better, at least in miles per gallon.

But lets look at gallons per miles.

At 25 mpg the couple burned 400 gallons over a year and their new 50 mpg hybrid cuts that to 200 gallons. They save 200 gallons.

At 10 mpg the family's SUV burns 1,000 gallons of gas a year. At 20 mpg the station wagon burns 500 gallons -- they save 500 gallons, much better than the couple."

Seeking better gas mileage? Think backwards - CNN.com