just to be clear, you save as much gas moving up from a 20 mpg car to 25 mpg car as you do going from 50 mpg to 100mpg. (think in terms of gals per 100 mile)
Car 1: instead of needing 5 gallons you only need 4 gallons. Saved: 1 gallon (20%)
Car 2: instead of needing 2 gallons you only need 1 gallon. Saved: 1 gallon (50%)
Not to mention that absolute savings are important when it comes to carbon reduction.