'66 Valiant: Gasoline guage acting abnormal

1966 Valiant Sedan.
What it is doing: Gas guage never reads 100% full, even when completely topped up. Seems like it is somehow off adjustment, since it seems to stay in the "low tank" and "empty" ranges for longer.

What I have tried:
- New sending unit (this was replaced before it began acting up, and worked perfectly fine for some time)
- New fuel tank
- New wire running to sender
- Replaced all fuses
- Replaced circuit board of cluster
- New cluster AND main voltage regulators
- Cleaned all grounds as well as wired in a master ground
- Sending unit is grounded
- *MIGHT* have replaced the Gasoline guage, but I cannot remember

Any thoughts? If I recall more that I have done, I will post. I've had my hands on most every nut and bolt on the car, so have the capacity to fill in needed details.