[jarendall]

factory built cars have a voltage regulator for the instruments and instrument lights, this keeps a constant voltage to them at all times, you most likely do not have such a voltage regulator, neither do I, this will cause the instruments to read different under varying voltage loads, most instruments are designed to work under certain voltage, i.e. 10 to 14 volts, check with the manufacturer of your instruments and see what the voltage ratings are for the instruments, you may have to install a regulator for them, or your alternator maybe swinging to much, it has a built in regulator