Place the ignition switch in the off position, and connect the terminals of two, series connected, flashlight cells to the gauge terminals. The 3 volts should cause the gauge to read approximately full scale.
If the gauge unit is inaccurate or does not indicate, replace it with a new unit.
The sending unit can be tested by first making a gauge unit test to determine the accuracy of the instrument panel gauge.
If the gauge unit is inaccurate or does not indicate, replace it with a new unit. If the gauge unit still indicates improperly or is erratic in its operation, the sending unit or wiring to the sending unit is faulty. Repair the wiring or replace the sending unit.
Empty 73 Ohms
Full 8-12 Ohms
More info at: http://www.mustangandfords.com