Fill your gas tank to the top and write down your mileage (save it). Drive you car until you have about 1/4 of a tank of gas left. At the pump write down your mileage and fill your tank up to the top again.
Subtract your start mileage from your finish mileage. Take the number of miles you drove and divide it by how many gallons of gas it took to refill your tank to the top. This will tell you how many miles you get to a gallon of gas.
Beginning mileage; 90,000 miles - Finish mileage; 95,200 miles Gallons used; 20 gallons
200 miles 200 miles divided by 20 gallons = 10 miles to the gallon