Classic/Antique Car Repair: 66 mustang, gas gauge, voltage control

Question
my gas gauge is broken, any advise on where to start without replacing entire system?

Answer
Disconnect the wire at the sender unit at the tank. let it hang free. The gage should read zero or empty. Then ground the wire and with the key on it should read full. If it does that then the problem is the sender unit. If it does not then the problem is the gage or the constant voltage control behind the instrument panel. That can be checked with a voltmeter. it should send 6 volts or so the the gage.
Brad