On the more practical side, GMC used all 6 volt fuel gauges in the '55.2 trucks 6 volt or 12 volt. They put an external resistor of 30 ohms between the ignition switch and the fuel gauge on the 12 volt models.
The statement about the fuel gauge being voltage independent is true ONLY for a limited range. The power (heat) created in the gauge goes up with the square of the voltage. i.e. double the voltage to the gauge and it generates 4 times as much heat.
If you increase the voltage enough, the gauge will likely fail by a conductor or connection burning open. In other words, you'll let all the smoke out and then it doesn't work any more. (Thanks Stuart)
