Originally Posted by CGameProgrammer

Constant changes in speed will kill mileage. Stopping, then accelerating, then stopping, etc., is really bad for fuel economy, so that's why you get 31.5.

Mileage is calculated by taking the miles traveled on a tank and dividing it by the gallons of gas needed to fill up. If you start with a full tank of gas, travel 500 miles, and refill the tank, and it takes 15 gallons to fill up, then the mileage is 500/15 = 33.3 mpg.

By the way, nobody has 50 mpg over a tank with this car, just over a short duration in some cases. Me, I got 35.3 mpg for my first tank and 37.5 mpg for my second. My third will have more city driving and may be 35-36 mpg.