1. Fill up car and write down starting milage 2. Drive car until it is down to between 1/2 and 1/4 tank 3. Fill up car and write down finishing milage and how many gallons (to the tenth such as 15.7 gallons) it took to fill up the car 4. Subtract finish - starting milage and divide the answer by the number of gallons it took and you have your gas milage.
