I just bought a 2009 Prius and am having the same problem. Only filled it up twice, first time took 7.6 gal, second time 7.1 gal. I'm not happy about this at all. The service dept of my dealership was clueless about it (or pretending to be in ordered to sweep it under the rug). This is very bad engineering. Surprising considering how well the rest of the car was thought out.

By the way, for those of you who are trying to ascertain your MPG on a tank of fuel from the multi-function display - you're wasting your time. The MFD is only a computer estimation. The ONLY accurate way to calculate MPG is by dividing the total number of miles driven on a tank of gas by the number of gallons to fill it back up.