After reading through these posts I just wanted to make a few comments. It has been suggested that the only way to know for sure is if the MFD rreading is correct is to ensure that the tank was filled to the same level before and after. While this is certainly true, it is very hard to do given the variables of temperature and bladder temperment while filling.

I have made calculations such as Wayne's a hundred times (that is why I developed a spreadsheet for it) and I have found after filling the tank that the calculation is quite accurate. In fact, over 47000 miles of Prius fills I can say that the MFD has never been off by more than 5% from the gallons pumped. Over that period, the average has been a 1.5% MFD overstatement. I would say that with the data I have collected, IMHO, it is more accurate to apply a 1.5% penalty to the MFD than to base a conclusion on how many gallons it took to fill the tank. That is because I have 90+ tanks of data that serve to smooth out all of the individual errors associated with filling one tank.

It's a Wonderful Day for Science!