Gas gauge way past full

If it's after market it may be the unit , check for good ground

This is a bad power wire going from the sending unit in the gas tank to the gauge on the dash. A bad ground would make the gauge go to below empty. Hope that helps! Jim

On a GM that’s indicating an open circuit, usually it the sender but have seen connector problems at the sender assembly. Just make you test the entire fuel gauge circuit.

On most older vehicles the gas gauge sender was basically a reostat that send more voltage to the gauge as the tank got full. Do they still work on the same basis? If so, that would make me believe that it is getting way to much voltage making it go way past full.

Not anymore they use a magnetic frequency to set the gauge instead of a rheostat and pulsating voltage from an IVR.