This seems to be a problem with repop senders, but it doesn't mean it's the ONLY cause
You could have a problem in several areas.
1 Wiring problems, IE resistive connections from the sender (and tank ground) up through the gauge, VR, harness, and power
2 Gauge itself could be out of cal
3 VR could be outputting improper voltage.
One way to check all this is to get down to RadShack and buy some 1 or 2 watt resistors, or combinations of resistors, to make up test resistances. This is what was in the old mopar gauge test tool.
The resistances for either temp or fuel (and I guess optional oil, some models) are:
L = 73.7 Ohms (empty)
M = 23.0 Ohms (1/2)
H = 10.2 Ohms (full)
Something I found on Mopars was somewhat different:
The resistor values are 78.5 ohms for the low reading, 21 for the mid reading, and 10 for high.
BUT you said:
Seems to calibrate out the proper ohms .
Just how did you determine this?
You should be able to drain the tank down to MT and read the sender with a meter, but make sure the car is level. Then fill the tank and take THAT reading.
Six gallons is an awful lot for an "empty" reading.