Ford doesn't use a linear fuel guage. Thats why it says you have a full tank for the first 150 miles (half the tank), then starts dropping like a rock, they want to make you feel like you are getting better fuel economy.
The guage itself usually doesn't have any smarts in it, but the circuit that controls the stepper motor might have a short, or the stepper motor itself might be going south.

96 Ford Contour SE MTX, SVT exhaust, KKM intake, SP Motorsports slotted rotors, KVR pads, SP5000s -Stereo sucks