In practical terms it's not that clear cut. Let's say I have an old car that gets 14 mpg and I'm considering two new cars, one that gets 33 and one that gets 50 mpg.

If I drive 500 miles a week, either vehicle is going to save me at least 20 gallons a week in fuel. But going from the 33 to the 50 is only going to save me 5 more gallons. However to get 50 mpg I probably need to buy an expensive new hybrid, whereas it's not too hard to find a good used car that will get 33 for a fraction of the new hybrid's purchase price. Then you have to consider cost of financing and insurance (both more for the new car). If I go for the 50 MPG car I'll probably never make up the difference in fuel savings unless I drive a LOT of miles.