Way to go Dave! Fixing grounds, even if it doesn't solve the problem, is always a good thing. I don't know your truck, but is there any way that the wiring to the gauge could have become reversed? Just a thought. 1966 - 1967 is the change point for 30ohms to 90ohms. I wonder if you have a 90ohm gauge and a 30 ohm sender? I doubt it, though. It really sounds like something is reversed. If you have an aftermarket sender, it COULD maybe be the wrong polarity. There are both positive and negative ground senders. I can't think of anything else right now. Good Luck!
Les
