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?
Its going to be the sending unit inside the tank you might be better off taking both tanks apart and piecing them back together to make one functional tank, you might also try just checking the electrical connections and look for severed wires from atop the tank