Fuel sending units read WRONG time after time!

is your fuel gauge stock and never changed?

if you are using an aftermarket gauge, just know that chevy / ford use different ohms for the tank sender, so an aftermarket gauge will read wrong every time unless it's one made for a mopar sending unit in the tank.

if you are using the stock dash gauge, something could be wrong with it.
you tested it out of the car, but that is using external wiring. maybe the wiring harness behind the dash has some kind of short...