Copied this from the Web.
The 1969 Dodge/Chrysler gas gauge sending unit varies it's resistance between 8 ohms to make the gauge read empty and then go up to 72 ohms to make the gauge read full
The guage are thermo electric, the lower the resistance the the hotter the heater in the gauge gets, the more the needle moves as in full.
The higher the resistance the lower the heater gets the less the needle moves, as in empty and the same reason the needle sits at or below E when the key is off.
10 = full
23 = 1/2
73 = empty
It's not linear and your guage will not read correctly with the stock sender. I do know if autometer makes a specific sender for our cars. To match their gauges.
Also in the Factory Service Manual it outlines the test procedure and the ohm range for full and empty with a +/- range. Free FSM mymopar.com