This has been bugging me for several years; every time the light switch is "ON", the fuel gauge level jumps up almost 1/4 tank.

I have a Getty 5 gauge dash: Clock, Speedo, Tach, Oil Press/Temp, and Fuel/Voltmeter.

All the gauges share a common ground and common voltage source for the gauge lights; I'm wondering whether one or both need to be isolated to prevent the interference?

Need some electriccal input here...

When I put a Subby in my kit car I ran inti the same problem, it turn out to be the
battery lead from the fuse to the battery was dirty. I fix my problem by running
another battery lead to the fuse block and supplying battery to the gas gauge. The
real problem is the voltage drop from the battery to the fuse block when the headlights are turn on.

904svo,
you hit the nail on the head! With these cars getting older, the problem will become worse.
Tom

904svo,
you hit the nail on the head! With these cars getting older, the problem will become worse.
Tom

You have a bad ground.

When the lights are on, more current flows through the ground path from the light switch to the battery.
A high resistance in the ground path means that the extra current causes a bigger voltage drop in the return path.
This means that the gauge sees a smaller voltage from the sensor because the voltage drop in the ground path is shifting the reference voltage up.

Thanks for the inputs: should the Fuel/Voltmeter gauge have a separate ground and separate 12v, instead of the existing series of grounds and power wires ?

My car exhibits the same phenomena. I'm just always glad to see that extra 1/4 tank show up, even if it's not real.