I've encountered at least three different AD trucks over the years that had dirty contacts in the ignition switch which would allow all of them to give a higher than normal reading on the gas gauge. On the first two trucks I took the ignition switches apart to clean the contacts. On the last one I just turned the switch on and off a bunch of times with the key which did the trick. You can check the switch for continuity with a meter looking for 0 ohms. The dirty contacts also cut proper voltage to the ignition circuit.
