It is either the sender unit in the fuel tank, the wiring for that, or the guage itself.
Some have the sender inside the fuel pump, which kind of means replacing the whole pump.
Check the wiring first make sure its not corroded and is secure.
If the wiring is ok, then chances are its the sender or the gauge.
Only someone who knows what they are doing can adequately test the gauges.
You should take it to an auto electrician many will test it for free give you a quote on fixing it, others will charge a fee just to look at it.
Try and find one who will check it out for free and give a quote so you know what you are up for.
You need a new fuel pump
Your fuel sending unit is not giving the correct reading to your gauge
Either the gauge is broken or the sender is broken.
fuel sending unit broken wire or gauge is bad
On a car, there is a float inside your gas tank. It is attached to a switch that sends the empty.....full signals to your fuel gauge.
you should go to a car shop for that one!:)
check your ground wire to the gauge
The floater inside your pump is sticking. You will have to replace your fuel pump in order to fix this problem.
The fuel gauge could be getting improper signals from the fuel sensor. The sensor is located inside of the fuel tank.
In most cases the fuel gauge sending unit in the fuel tank has failed.
Most Likly a fuel sender problem
fuel sensor in the gas tank is bad or the wires to it are loose or broken.