I have been getting to know my Prius the last week and a half. Excited with my display's 39.1 mpg I thought I would top off to add a tank to my mileage log today. To my horror when I do the math of 148 miles and 5.374 gallons my calculated FE is 27.5 mpg! I know it has been cold and I have been not the best hyper miler transitioning from a manual to an automatic, but I am really bummed! I am hoping that the dealer really didn't fully fill the tank and that in two weeks when I top off again I will get a more accurate mpg. Can the computer in the Prius be off by so much?