Go to www.westach.com. They have 6 volt, positive ground water temp, oil pressure gauges and 2 inch, 3 inch and 4 inch 6 volt tachometers. Very good products. They will build the gauge to order. Very good company!!!!!
Gus-
Except for the bulb that goes in the gauge to illuminate it at night, most tachometers do not really care what the voltage of the system is, or the ground. Don't get too hung up on that.
Mechanical temp gauges do not use any electric either, except for illumination.