The best way, and assuming the display error is actually a constant, is this: Over several consecutive fills, record the volume the display claims you burned, and the volume the pump says each time. Add them up and divide claimed total gallons by pump total. This will give the true error factor, unlike figuring the error factor for each fill and then averaging those numbers. This works even if there are partial fills in the middle, as long as the starting and ending fills-ups are reasonably complete and consistent. By the above method, my error factor has been very consistent ever since I got the car. Apologies, and never mind, if you're already doing this.