You need a fuel level sending unit. Sometimes they can be removed and cleaned up to work again. The fuel tank needs to be removed. You can remove the sender and connect it to the harness. Move it around with the key on to see how the dash gauge reacts. There are little fingers that rub these lines that look like a piano key board. Sometimes they get dirty or loose connection.
In my personal experience, doing this helped and makes it around 97% accurate.
First thing is most of the time the float will slosh around due to water, particles, and contaminants. The float
will look full when its not or vice versa. Run a good water cleaner like Iso Heet Red Bottle through a quarter of gas. Next take out the dash cluster and slightly manually adjust the fuel needle so it is lined up exactly to the red line. Hammer the inertia switch in the back once or twice then reset it. Finally, do a flood clear start and dry start the car by holding down the gas pedal and starting it a few times. Doing these things fixed this issue.