Our 64 starfire gets light driving duty, and not much at night. But I saw this week that when the lights go on, the gas gauge drops about 25%. Lights off, the needle climbs back to where it was. I've had several 60's GM cars but I've not seen one do this. Scared me silly because I was only at quarter tank anyway, and when i looked after lights on, it was bottomed out. I filled it up, and then I noticed it. Lights go on, gauge fell to 3/4. Weird.
The gas gauge is really just measuring resistance, right? So something about the lights going on is changing that value, but I don't get why it would affect the gas gauge circuit. Some cars by now look like a slasher movie under the dash, but not this one. Every wire is as manufactured. Just getting old is all.

I could swap out the light switch, I do have another around here. But what would a few good tests be?

 

Check your + and ground wire connections at the tank. These are all old and maybe dirty enough to prevent a solid tank signal to the gauge.

Headlights draw a lot of amps. To the point where even the switch can get hot if it’s old and/or dirty. One of my switches way back way got so dirty it would short out on high beams and I’d lose all headlights. At night on country roads... I drove on low beams for a long time before I knew how to fix it.

Consider pulling the headlight switch and cleaning it with Deoxit D5 or similar contact cleaner as a possible quick fix. Same for the high beam switch.

Here’s what I use:

If you get Deoxit, also consider cleaning your bulb contacts and the connector contacts to make a better (and maybe lower draw) connection.

You’ll need a tiny flat blade screwdriver to release the wire brass harness contacts from the plastic connectors. At least the bulb spades are exposed for easy cleaning.

To clean electrical connections I generally do a light pass with a dremel polishing tool, then detox, then reassemble.

hope that helps
Chris