Set the trip distance on your odometer to zero the moment you leave the gas station with a full tank. The next time you go in for a fill, fill the tank, but don't top off. Write down how many miles are on your trip odometer as well as the gallons you purchased on the gas pump meter. Divide the miles by the number of gallons and it will give you your approximate gas mileage since your last fill. Keep a log of your mileage per fill for 10 fills. Add these numbers together and divide for your average MPG.