SL,
First, you might check your actual tank specs online. Just to make sure.
Second, show your gallons used on the UG. When you fill up from a previous fill up, you can then see how accurate the UG is... there is a way to calibrate your UG, which I've already done and found the default UG settings to be under-reporting fuel usage by about .058 percent. For relative accuracy use the same pump. You should also calibrate your miles. Then you will be getting a accurate actual fuel usage.