Ha ha... ya, don't I know it!! I admit that I am totally guesstimating how much fuel I've used so far based on the general neighborhood of where the gauge is pointing... when it more or less wanders between the full mark and the half way mark, I'm guessing the tank is roughly 3/4... and so on. I kinda panicked the first time I took a freeway onramp and my fuel level "dropped" from about the 7/8 mark to the 1/2 mark
... of course it slowly came back. Getting more used to it now.
I think my instrumentation work is going to have to include a more accurate fuel gauge. I was thinking that the MPGuino code could be modified... since it already has the ability to measure how much fuel is being used instantaneously, and since the car manufacturer provides the fuel tank capacity, it ought to be a fairly straightforward modification to provide the MPGuino with the tank capacity (minus some small amount if you want a safety margin) so that it can continuously subtract the instantaneous amount used from the total amount and then display how much is remaining in the tank (for instance, via bar graph LED). Each time you fill up and hit the tank reset, it starts over.
I do enjoy open source projects for that reason ... !