You will first want to make sure you start with a tank on empty. Next, fill your gas tank and write down how many gallons it took to fill it, for this example lets say it took 12 gallons. Now, write down what your odometer says your mileage is. Of course now you will drive as you normally do until you are almost on empty again, write down your mileage and subtract the first amount from it. For examples sake, lets say we ended up driving 264 miles. Now take 264 and divide it by 12 (because that’s how many gallons of gas we put in), and now you have your answer for you gas mileage. For our example, we got 22 miles per gallon.