I just recently bought a old beater truck for $175 dollars. It runs great, but I have to guess how much gas I have by using the old slosh-and-listen technique. This truck has two fuel tanks. When I switch the tanks the one tank that doesn't work reads correctly on the guage. On the tank that does work the guage pegs itself way past the full mark.Quite annoying. I checked all the grounds just after I bought the truck and everything was in good shape. Should I replace the guage? What else could it be?