To roughly estimate how many miles per gallon you get, you will first want to look at your gas gauge and note where it is at, next put some gas in your tank and also make a note of how many gallons you put in. Now, write down the mileage that is on your odometer. You will want to drive as you normally do until your gas gauge is back to where it was before. Now look at your mileage on your odometer again, subtract your first mileage number from this number. Once you get your answer, divide that by the number of gallons you put in your tank. This will give you your miles per gallon.