Get a spare sending unit. Hook up the wires and move the float arm, noticing the needle deflection. This will tell you if it's the sending unit or the gauge. I'll bet, sending unit. They go bad from water condensation in the tank, and rust. Also the brass floats can develope a hole and get flooded. If you need an entire sending unit, and can't find the right one for your tank, fear not. I soldered and wired together the parts from a totally different sending unit on mine and IT WORKS.
