Can't Figure Out What's Wrong With My Gas Gauge

[SavageC20]

I have read every gas gauge thread that I could find, but I am still having trouble figuring out what is wrong with my gas gauge. The gauge reads approx. 1/3 tank when FULL. The needle will go down as the level in the tank gets lower...all the way to empty.

I have checked continuity on my sending unit wire and it is fine. I have checked the resistance of the sending unit and various gas levels in the tank. I know its supposed to be a 0-90 ohm scale, but I topped off the tank and it was reading 100 ohms?? Could be my cheap multimeter...not sure

I pulled the fuel gauge out of the cluster and tested the resistance across each stud and compared this to the readings given in this thread:

http://67-72chevytrucks.com/vboard/s...d.php?t=640615

My readings are shown in the picture below (sorry for bad diagram)

The other pictures are from the thread listed above

I also pulled the resistor off the back of the gauge and tried to check resistance, but I was unable to get a reading. Does this mean the resistor is bad?

The previous owner installed two saddle tanks on the truck and I don't know if they are wired into the gas gauge somehow. I don't believe they are because the wiring on the back of the gauge/cluster looks untouched. Both saddle tanks are currently bypassed and the electric transfer pump is completely disconnected.

Where do I start?