Possibly the sending unit, possibly the combination meter, possibly the wiring between the two.

http://www.nicoclub.com/archives/inf...e-manuals.html

Section FL for fuel pump removal, section DI page 19 for TESTING the fuel sending signal with a digital multimeter.

That's a good place to start, unplug the fuel pump and measure resistance on the pins shows with a FULL TANK to verify the proper 3ohms resistance. If the fuel pump is sending the correct signal you can test for the same resistance reading at the combination meter harness (check FSM for which wire), if the combination meter is getting the correct info you probably need a new meter which is covered in the DI section.