Carl Bialik from the Wall street Journal has a great article about how many people do not understand the correct choice for their cars in order to maximize gas mileage.  It's not that people are stupid or that the calculations are difficult - it's that the math is counterintuitive to what one would expect.

Assume you drive two cars each 10,000 miles per year. One car is a sedan (20 mpg), the other car is an SUV (10 mpg). You'd like to buy a new car with better gas mileage order to decrease your overall gas usage.

Option 1: Replace sedan with a super-hybrid electric car that gets 500 mpg - keep the SUV that gets 10 mpg
Option 2: Replace SUV with another sedan that gets 20 mpg - keep the other sedan that gets 20 mpg

Contrary to what you might think, Option 2 actually uses less gas. The numbers obviously change if you have different usage patterns for each car (e.g., SUV 5,000 miles per year, sedan 10,000 miles per year) - but it's still a pretty counterintuitive calculation.  

Professor Rick Larrick from Duke's Fuqua School suggests replacing MPG with gallons used per 100 miles. This would make the gas mileage calculation much more intuitive for consumers. It would also make cars with very low gas mileage look even worse than they currently do, which is why it probably won't ever be implemented.