Is your iank full? You could have a grounded place in your wiring, you could have a defective sending unit in your tank, the tank might not be grounded, or you could have a bad gauge. go to tank and unhook the gauge wire. While someone watches the gauge with the key on, ground the wire if gauge reads opposite when you ground wire then your wiring and gauge are ok. Then you either have a bad ground on your tank or your sending unit is bad. You could start at gauge and unhook the wire from the sending unit then jump that gauge terminal to ground. If you see the gauge reads opposite then check as above.
