I am working on an antique car that has a 6 volt positive ground electrical system with the exception of the fuel gauge which is a 12 volt negative ground. Currently the car has a voltage converter that takes a 5-11v input and converts it to a steady 12 volt output. However, the -6v supplied by the cars electrical system results in an output of only 5.65 volts from the converter. If I supply +6v (external adjustable DC power supply) the converter will output the steady 12v. I have found a few voltage converters online that state they convert a 6 volt positive ground input into a 12v negative ground output. The first has only 3 wires, the -6v input from the cars electrical system, the +12v ouput and a ground to chassis. The sending unit for the fuel gauge (variable resistor attached to a float) has 2 connections, one back to the gauge and the other to ground.
My questions: Can the sending unit be grounded to the car's chassis which also has a 6 volt positive ground electrical system connected to it? Can the two dissimilar voltages and polarities exist with a common chassis ground?