One's fuel costs rapidly diminish in a non-linear way as MPG goes up. The difference between 15mpg and 16mpg is much larger than the difference between 40mpg and 42mpg.
If we were logical creatures, we'd measure fuel economy not with miles per units of gasoline, but with units of gasoline per mile, or gallons/liters per X number of miles.
Cost to go 1,000 miles:
15mpg: 67g
20mpg: 50g (a 17g reduction from 15mpg!)
25mpg: 40g
30mpg: 33g
35mpg: 29g
40mpg: 25g
45mpg: 22g
50mpg: 20g
55mpg: 18g (a 2g reduction from 50mpg)
Once we get above about 40mpg, the differences get really small and the engineering sacrifices required to reach the next level of efficiency get even harder (e.g. vehicle capacity, acceleration, cost, complexity, aerodynamics versus ease of use).
It follows that the U.S. land of land yachts could cut its fuel consumption in half just by changing the types of cars we drive. Going from a 20mpg SUV/Prettytruck to a ~40mpg commuter car (like an improved version of a gasoline car like the Fit or a hybrid) should end all complaints about gas prices. Even at $10/gallon, 1,000 miles of transportation is still relatively cheap in a ~40mpg car.