Jim is correct. Some gauges have just a couple of coils that drive the needle and will not return to zero when power is removed. Others have a little more electronics behind them (some have a lot) which will allow a return to zero when power is removed. The real question is why does the gauge reads 1/2 tank when tank is just under full. Are you sure that the sender resistance is matched to the gauge? What happens if you drain some fuel from the tank?
