Here's how you figure out your car's actual miles per gallon (MPG):
When you fill up the tank, write down the number of miles on the odometer.
The next time you fill up the tank, write down the number of miles that are on the odometer this time.
Write down how many gallons of gas went into the tank too. (This number will be on the gas pump.)
Subtract the number of miles at the first fill-up from the number of miles at the second fill-up. This is how many miles you drove between filling the tank.
Divide the number of miles you drove by the gallons of gas you put into the tank the second time you filled-up.
The number you end up with is the number of miles your car drove on a gallon of gas: your car's miles per gallon or MPG.