Thanks everybody for your input and Jason for fixing my signature. Since my original post, I have seen a correction in my mileage. When I posted, I had about a 10 mpg difference between my calculated and screen mileages. On my next tank, I saw the reverse. I talked to the Prius technician at my local dealeship. He said that the Prius has a bladder inside the gas tank, which may be the cause of this difference. The bladder might have a fold in it or something else that does not allow me to use its entire volume, which can cause the pump to think that the tank is full when it is not. This can show up from tank to tank, but over time will cancel itself out.