There could be multiple reasons why your gas gauge is stuck on full or is even reading past full. Oftentimes, it could be that the fuel sending unit doesn't have a ground. Or, if there's an open circuit between them, or if there's a break in the wiring that leads from the gas gauge to the fuel sending unit. Another possible culprit is the gas gauge's resistor sending full voltage to the fuel gauge at all times. In stark contrast, the voltage reading should vary as the fuel levels change, thus altering how a fuel gauge's needle travels. As the needle goes from Empty to Full, the resistance in the circuit should increase. Therefore, you should carefully take a look at the circuitry or wiring connecting to the gas gauge to spot any issues.