I have been searching for help with the aptly described problem of filling the gas tank. To my amazement, I found on another forum that the Prius has a flexible bladder and not a tank! This explains a lot of problems with erratic fills. If it is cold, you can squeeze less into it. Also the "fuel gauge" is rather unreliable given the nature of the "tank". The message seems to be fill it as much as you can when one or two bars show, don't trust the gauge very much, and don't run out of gas. I learned this the expensive way as others here have! Colleen in Wisconsin