[Coley]

Did you ever have a dual (saddle) tank set up in the truck?
If so, there is usually a mechanical and sometimes and electrical switch to control the fuel flow.
Some of those were tied into the fuel gauge wiring so if you had the switch in the wrong direction the gauge would respond.

I had this happen on mine. I have a saddle tank that is long redundant and is no longer part of the fuel system on the truck but the electrical switch is still tied to the gauge.
If I flick that switch the gauge drops to zero......damn confusing until I figured this out.

Coley