Hey Lobo49,

The ammeter dosen't give a hoot about voltage and will work fine with twelve volts. In order to make sure that you don't fry the fuel gauge, you will have to install some sort of resistor on the fuel gauge. The Runtz Gauge Resistor from 5th Avenue is one such resistor and possibly the best one for the application. Please see the links sections of this site for 5th Avenue's contact info.

Good Luck With Your Project,

Jon