For those of you that have tried to convert your car to 12 volt and didn't know exactly what to do with you gas gauge, I hope this helps. You can order a Runtz type resistor from Speedway Motors or run to your local Radio Shack and pick up a voltage regulator. The installation for either of them is pretty straightforward. Input (12v wire originally connected to gauge), output (6v, goes to gauge), and ground (connect under gauge mounting screw). Voila! Your done!
Part number for the Runtz type resistor from Speedway Motors - 91064101
Part number for Voltage Regulator from RadioShack - 7805
I have used both of these, and I will say that the Runtz type resistor is nice, but if you don't completely wrap it in electrical tape it is possible to arc it off of something and it will fry the internals. There went $20. The cheapo regulator from RadioShack cost $2. and they are pretty much idiot proof.