Having done several 6 to 12 conversions, one thing I learned early is to not use a "resistor" style voltage reducer. They are nothing but junk and unless you perfectly match them to the load, you may get any thing for voltage. To do the job right, take a look at the solid state voltage reducers sold by Ron Francis Wire Works. He sells two versions...one small one that is just perfect for things like gauges. There is a large one with 15 amps capacity that can be used to power a heater fan or old style radio if needed. Being solid state units, they put out exactly 6 volts regardless of the load. (up to the rating of the unit, of course)
