If you want to calculate your own average gas mileage, you can take a few easy steps to do so. First, fill up your gas tank to “full” and take note of how many miles are on your car. Second, when it is time to fill up your car again, take note of how many miles are on the car and compare it to how many miles you had when you filled the tank before. That number will reflect how many miles you traveled since your last fill up. When you are finished filling the tank, look at the gas pump or on your receipt to see how many gallons of gas you used to fill the tank. That number is the number of gallons you used to travel the distance you measured earlier. Finally, divide the miles you traveled by the number of gallons of gas that you used. Your final answer will reflect how many miles per gallon of gas your car got on that trip. For example, if you traveled 200 miles and had to fill your car with 10 gallons of gas, your car got 20 miles to the gallon. The average gas mileage for a car stays around 25 miles per gallon, so you can conclude that your car’s gas mileage is not very efficient.