I have a couple of fuel gauges for the '55-'59 series GMC trucks. One is early '55.2 and is 6v, the other is later and is 12v. I measured the the resistance of the limiting and control coils in each and did a few calculations.
Rl = Resistance of the limiting coil (empty side)
Rc = Resistance of the control coil (full side)
Rs = Resistance of the sender = 0 - 30 ohms
6 volt gauge: Rl = 53 ohms, Rc = 30 ohms
12 volt gauge: Rl = 109 ohms, Rc = 30 ohms
The equivalent resistance of the sender and the control coil is the same for 12v and 6v. At the empty position the equivalent resistance is 0 ohms because the the resistance of the sender is 0 ohms. At the full position, we have two equal 30 ohm resistances (Rc and Rs) in parallel and the equivalent resistance is 15 ohms. We'll use Re for the equivalent resistance.
Maximum Current (empty gauge) (Volts/Ohms):
Imax = Voltage * (Rl + Re)
6 volt Imax = 6/(53 + 0) = 0.113 amps
12 volt Imax = 12/(109 + 0) = 0.110 amps
6 volt gauge on 12V = 12/(53 + 0) = .226 amps
Power in watts: P = I*I*R
6 volt Pmax = (0.113)*(0.113)*(53) = .667 watt
12 volt Pmax = (0.110)*(0.110)*(109) = 1.3189 watt
6 volt gauge on 12v
Pmax = (0.226)*(0.226)*(53) = 2.71 watts
I made this calculation years ago and based on the small amount of power (read heat) being generated decided that it would be safe to run the 6 volt gauge on 12 volts. I have run the gauge in my '49 truck many years on 12 volts without problems.
I suspect the leading cause of gauge failure is getting the source voltage onto either the sending unit wire or the control coil terminal on the back of the gauge. Most gauges have a red tag on the sending unit terminal warning against this.
