It is the GMC fuel sending unit that makes it possible for the driver to know exactly how much fuel they have left before they need to fill up. The days of using a dip stick or sight glass are long past. The fuel sending unit has been a piece of standard equipment on all GMC trucks since the 1940s. The fuel sending unit is the mechanism that is in the gas tank on each GMC truck. On most models the fuel pump is also attached to it. Not only does this device send information to the driver about the level of fuel in the tank, but also the means by which the fuel can be taken up by the fuel pump so it can be delivered to the engine in the front of the vehicle. There is a specific GMC fuel sending unit designed to be used for each model made by this manufacturer. Each of them made to fit a specific gas tank, thus the reason a specific unit has to be matched up with the model and year of your truck. The location of the fuel sending unit is in the fuel tank, and if you need to replace it then the fuel level in the tank should be at a minimum. This should be done so lowering and removing the fuel tank is easier and more manageable. It can be accomplished by siphoning the fuel out or just using the truck until the fuel level has sufficiently dropped to a safe level so it can be easily removed. The removal of the fuel sending unit is easy with the tank out. There is a simple retaining ring that is turned and the unit can be pulled out and replaced. Remember to replace the O-ring so a good seal is made when the new part is installed on your GMC truck.