1. Fill up the gas tank completely and write down the mileage on vehicle's odometer.
2. When it is time to refuel, fill up the gas tank, write down number of gallons it took to fill the tank and the mileage on your vehicle's odometer.
3. Calculate the distance driven.
4 Divide the number of miles by the number of gallons it took to fill the tank. The result is your car MPG.
More info: http://bit.ly/Calculating-MPG