The sender has two resistance wires and a float with contacts that slide over the wires.
With the float in top position the resistance is minimal, at the bottom position the resistance is maximal.


It is not possible that the resistance is lower than the minimum for a given float height. Higher resistance is possible if the wires are corroded. So malfunctioning of the sender will result in a lower fuel reading, never a higher. The same for the ground (located on the left rear wheel well), bad ground result in a lower fuel reading.


In addition there is a low fuel warning light..


In the instrument cluster the ground and power are shared with the coolant temperature gauge and the oil pressure gauge, are these behaving properly?


Ground is also shared with the high bean indicator light, you can check the ground by switching to high beam an see if the fuel gauge reading changes.

Rob