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I wrote this post  on one of the car forums I subscribe to:

Over here in the states, we focus on how much distance we can cover per unit of gasoline (e.g. miles/gallon). The way the U.S. government actually calculates it (say for the CAFE regulations) and the way Europe and other countries do it is by unit of gasoline consumed per unit of distance (e.g. gallons/mile). It’s very interesting to realize the understand the difference of lower and higher gas mileage when you see it this format.

(to calculate gallons/mile, just take the inverse, or 1 divided by your mpg)

For example, let’s take a 5mpg, 10mpg, 20mpg, 30mpg, and 40mpg car:

5mpg car = 0.2gallons/mile

10mpg car = 0.1gallons/mile

20mpg car = .05gallons/mile

30mpg car = 0.033gallons/mile

40mpg car = .025gallons/mile

You can see that it’s actually a larger gain in efficiency to get cars with lower mpg (<20) off the road than it is to go from a 20mpg to 40mpg car. Basically, you’ll save more money at the pump by getting rid of your 15mpg truck and going to a four cylinder SUV that can get 25mpg versus getting rid of your 20mpg minivan and going to a 30mpg sedan. And it really doesn’t save you much by going from a 30mpg car to a 45mpg hybrid. I thought this was very interesting when I heard it on NPR the other day.

Here’s an spreadsheet graph I made to illustrate it better:

Edit:

In conclusion, I think if you can double your EPA mileage, than that’s great. But most people can’t. And probably won’t on a daily basis. By increasing your mileage by 50%, you’re only going to save 25% on your gas costs. If that’s significant for you, than that’s great. But how many people can and will even put forth the effort to do that? I think in conclusion, hypermiling a car that already gets 25mpg or above is not as good as it’s said to be.