Edited: another fuel/temp gauge thread, but turns out the harness is melted together

You may have more than one issue. Your fuel gage acts like what happens when you replace the tank sender with an aftermarket unit. If the original sender has been replaced with an aftermarket version, they are very different than the original, and the gage will not work properly.

If that is your problem, you have two basic options:
1. Get an oem style sender
2. Get a device called a Meter Match that can be programmed to match the aftermarket sender to the fuel gauge.