I was having the same issue with the aftermarket sending units and aftermaket fuel gauges. The first guage, an Equus brand that was supposed to be for Chrysler read backwards, but was reasonably accurate. Next I tried a speedhut gauge and programed it to the sender unit and it would have the rapid gauge drop from 3/4 to 1/4 in about 30 miles of driving. Then I redid the dash gauge cluster with an Intellitronix 6 gauge cluster that has a fuel gauge in it. Hooked everything up and the fuel guage read 38% were the speedhut guage was reading zero. Couple of test drives and the gauge was down to 25% put 8.50 gallons in the tank and the gauge went to 85%. Not enough testing time yet as we are on a Covid-19 stay at home lock down here but it looks promising.
