I have a 69 The gas gauge arm will go down to less than 1/4 and no further, Never felt comfortable with that never wanted to find out how much was left!! I filled it up the other day. When I started the car up gas gauge went to like 3 o’clock way off scale. Have driven couple hundred miles and gauge still at 3 o’clock ???? I’ve checked grounds etc. shacking the car, tapping tank with rubber mallet.connection at back of gauge in dash. Missing what, Just time to replace?

 

First, open hood and disconnect coil lead from coil - terminal.
At trunk, under latch, look for a tan wire going through floor, as it goes to sending unit, and disconnect connection.
Using ohmmeter, one lead to ground, a really good ground, and other lead to tan wire in floor.
This action should show some ohms, 90 ohms = full, 40-55 = half tank, and lesser ohms, lesser fill, closer to empty.
If no ohms, remove and clean ground lug on front of member pass side, and wire brush the bejesus and reconnect Coat with antiseize preventing rusting.
The above has to work, first and foremost. No worky, replace sending unit. If replacing, duct tape wire on top of tank in a valley, preventing wire from getting crushed from tank-to-floor interface.
If sending unit works, then ground wire lead in harness as this action would be an empty indicator and gauge to go to empty, hopefully with IGN on.
Also, with body lead, connect a 40-60 ohm 1/2 to 1 watt resistor between lead and ground. This would be an estimated half tank indication.
Measure power at disconnected wire to ground, should be a value less than battery. Have voltage, good, remove gauge and shake bringing back needle to its normal zone.
Good luck.