Could be a number of things
1--repop senders don't have a good history of being accurate
2--the gauge unit could be incorrect. "How old," again, are these cars?
3--the inst. limiter could be off as well. You may not notice that the temp gauge is also wrong
You can check the gauge for accuracy with resistors, 10-15, 25, and 70-75 ohms. Make up combos at RadShack. For example, buy a four pack of 100 ohm, 1/2 watt and wire them all four in parallel. This gives you a 25 ohm resistor for checking either the temp or fuel gauge at 1/2 scale.
You probably are gonna have to bend the new sender to get it accurate at MT, and deal with the other end being off. If I was in an area with a bigger Mopar population, I might be able to deal with this problem.