Need fuel gauge help, very confused

[SS427]

I am pretty good with wiring but this one has me totally stumped. I always check every system on a car when I am done restoring to it, this one being a 70 Camaro.

Whenever I put in a new tank and sender I always check the sender before I install it in the tank and then again after its installed. This one tested perfect. 50 ohms at just over half a tank and 90 ohms with a full tank. This was all done manual with no fuel. Check

Install the tank in the car, the dash and all wiring. Turn key on, tank reads empty. Add 10 gals of fuel and gauge now reads just over half. Check

I then unplug the sending wire from the rear harness at the trunk. Gauge buries the needle to the past full mark. Check

Ground that same sending wire with a jumper wire and it stays buried to the right. Huh? Nothing I do can I get the gauge to read empty by grounding the sending wire.

The driver's side terminal of the gauge reads approx 12 volts and the passenger side reads 5 volts. Check. I also put a jumper wire from the ground terminal stud on the gauge to ground just in case I was not getting a good ground there (though the gauge would not have read correctly if it wasn't) but I knew it was.

Anyone have any idea what the heck is going on here? Yes the gauge reads fine when hooked up normally but I cannot get it to read empty when I ground the sending wire and for that reason I want to make sure I do not have another issue somewhere. Come on electrical gurus. Please help