It could be as simple as bending the float arm, the installer may have it bound up in the tank. Start by pulling the sender out and move it from full to empty and see what your gauge reads. If it's still off, hunt down the information on resistance and ohms of the gauge and sender as mentioned above.
I would drop the tank before pulling the bed if you have nice paint. If it is rough, pull the bed, a full tank of fuel will weigh quite a bit.
Joe
