Gauges all read incorrectly and vary with signal

[Rick Reid]

either a bad ground or a dirty (bad) hot wire

 

[Schurkey]

Gas gauge reading "more full" than true fuel level is epidemic, and almost always caused by high resistance in the circuit.

From instrument cluster, to gas tank sending unit, to the sending unit ground, every corroded connection or corroded wire makes the gauge read a little bit higher than it should. A broken/cut wire (infinite resistance) will put the gas needle way past full.

I'd start with the gauge cluster ground at the metal tab where the diagnostic (ALDL) connector is mounted.

Thanks for the tip about the instrument cluster ground at the ALDL. I'm gonna check mine tomorrow--my voltmeter has always read lower than true system voltage.