First, fill your tank up to the first click on pump. Write down your mileage. Drive it for a few days. Then go fill tank again to first click on gas pump again. Write down your mileage then, and how many gallons you put in your tank. Then subtract how many miles you went. ( your new mileage from the old mileage) Then with a calculator divide miles that you went by the gallons you put in your tank. This will tell you a roughly how many miles you get to the gallon. Hope this helps.