It is the fuel gauge or the sending unit. The fuel gauge is on the instrument panel and the sending unit is in the fuel tank. A properly equipped shop can test the gauge but it is difficult to test the sending unit in the vehicle. That being said... The sending unit is most likely the issue due to age. You will need to remove fuel tank to replace sending unit. The fuel pump is attached to the sending unit and we recommend replacing the whole assembly as a unit. This will be a difficult job to do yourself safely due to having to get the tank out from under the vehicle and fuel lines being opened. If you decide to do it yourself do not cheep out on the part. Pay more for the high quality part because you will not wish to do it again.