the fuel sendine unit is nothing more than a variable resistor. You need to measure the resistance (Ohm) of the fuel sending unit when the fuel level is low by disconnecting the electrical connectors and measuring the the ohms between the two terminals you can compare this to the factory specification. Also be sure to make sure the float is not stuck. Once you have verified the fuel sending unit is within spec you can measure the resistance in the wires from the fuel sending unit to the gauge. If the wires check out you can assume the gauge is inaccurate. There is a way to actually check the fuel gauge with a resistance meter, but few people have one. Hope this helps, you will have to refer to the manual for resistance specs.