When you fill up the tank, stop on the first click of the pump (the pump clicks and shuts off near the maximum capacity) and do not add anymore fuel. Write down the total gallons of fuel used and the mileage on the odometer. The next time you fill up, repeat the fill up procedure (stop on first click). Again write down the gallons and the mileage on the odometer. Now subtract te previous mileage from the current mileage and divide that into the number gallons you have just pumped. The answer is miles per gallon. If you keep track for a few fill-ups you will get a better miles per gallon average.