A fuel level gauge is a two part system composed of a means to measure the fuel levels present in the truck's tank which is called a sender, as well as a gauge which displays what the sender measures to the driver of the vehicle.
More on Fuel Level Gauges - Diesel Trucks & More
The sending unit of a fuel level gauge is actually positioned within the fuel tank of the vehicle. It is made up of a float made of foam which connects to a thin metal rod, the end of which is then mounted to a variable resistor. This device actually resists the flow of electricity. What this means is that as levels of resistance increase, less electrical current is able to pass through. A piece called a wiper connects to the gauge and slides along the aforementioned strip of material; allowing current to flow from the gauge to the resistor. Inside of the sending unit, the float will begin to drop once the fuel has dropped below a predetermined position. The floats upwards movement is limited by the top of the fuel tank or the rod it is connected to. The needle on a fuel level gauge will not begin its movement until the float starts to sink, which will occur when the fuel level dips below the level of the float. The sender shoots a current through the resister which wraps around or is placed near a bimetallic strip which fastens to the gauge's needle through a linkage system. Less current will be able to pass through the heating coil as the amount of resistance increases, which cools the bimetallic strip. When this strip straightens out during the cooling process, it will pull the gauge's needle from the full side to the empty.