Years ago, my GM fuel gauge was subject to incorrect readings like that when the thin film resistor that was printed on a ceramic "bar with two gauge terminal stud holes in it" failed open. If you can access the back side of the instrument panel, examine the configuration of the fuel gage design. Unlikely to be the same as the old tech that I described abive, but you never know.
Of course, what would help a lot would be to know what the ohms value should be for a full tank (if the fuel level sending unit in the tank is a variable resistance sender) and see what it reads.
The float on the arm in the tank if it is that type can some times get saturated with fuel and sink, but a mid tank reading at full seems unlikely for that circumstance. These days with tech increasing, they might have gone to a sensor that no longer uses the familar float arm and variable resistance wiper potentiometer sender.
Dropping the tank to pull the sending unit can be a chore, hope you can get fixed up without having to do that!
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