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."