love2fixit,
My previous post is way out in left field. There is a very simple procedure to check the dash gauge and the wiring back to the tank.
Put the gauge back in the dash and connect it properly..then, turn the key ON and pull the wire at the tank sending unit. The dash gauge should read FULL.
With the key still ON, ground that same wire you pulled at the tank to a known, good clean ground. The gauge should go to EMPTY.
If this happens, the gauge if good as well as the integrity of the wires back to the sending unit.
This points the fickle finger at the sending unit or the sending unit to ground path.
I think this is the best starting point. Either both your gauges ARE bad, or something is clouding the troubleshooting. The above test has been outlined dozens of times on this site and narrows the field as to which component is screwed up.
Stuart
