You want to know the MPG you got between fill ups. The last one and the current one you just did. You put in the miles you driven some how, there would be two options for this, using the overal odometer reading minused by the previous odometer reading from the last fill up or the trip odometer. Then you divide that result by the current fill up. You have to you the current fill up because that will tell you how much gas you have used.