The fuel gauges in our old trucks use two coils (electromagnets) to position the needle.
One coil (called the limiting coil) pulls the needle to the left or empty position. The other coil (called the operating coil) is positioned at 90 degrees to the limiting coil and pulls the needle towards the full side.
Current from the ignition switch, enters the limiting coil when the switch is in the 'on' position. The other end of the limiting coil is connected to both the operating coil and the tank sending unit. The other end of the operating coil is connected to ground.
If the gas tank is empty, then the resistance of the sending unit is near zero ohms and all of the current from the limiting coil goes to ground through the sending unit. With all of the current flowing through the limiting (empty side) coil then needle is pulled to that side.
If the gas tank is full, then the sending unit has about 30 ohms of resistance and most of the current from the limiting coil will pass through the control coil to ground. This current will cause the needle to be pulled away from the limiting coil (empty side) and towards the operating coil (full side). The operating coil has more windings than the limiting coil so it takes less current to generate enough magnetic force to overcome the limiting coil and pull the needle to the full side.
Since the needle is controlled by the currents and resulting magnetism, the fuel gauge operates on current and is independent of the voltage supplied to it. If it wasn't, then the fuel level would 'change' every time the ammeter indicated a discharge or the system voltage changed depending on the speed of the generator and the charge in the battery.
I also have run the fuel gauge in my '49 on 12 volts since the mid '70's without problems.
