Hey there I just thought I would hop into this discussion.

I talked with the mechanic at the Toyota dealer here when I was taking my 2004 prius in for another problem (which I will talk about elsewhere) about the gas gage issue.

I had two bars of gas in there and he told me to fill up the tank. I responded that the there was still a lot of fuel in there and he said I should fill it up when there are two bars as the computer will think that the car is running out of gas and can give error codes. I'm not really sure if this is really the case but it does say in the manual (p. 114) "it is a good idea" to not let the tank level fall below 1/4 of a tank. No real explanation of why this is the case, however.

Christian