2001 GMC Sierra with a 5.3 L V8 and 132,000 miles:
I fill up my truck and the fuel light (when full) stays on empty until the truck tank gets about 3/4 full. Then, it will ONLY register the fuel level when moving (or shaking) the truck. When stopped at a light or parked, it will drop instantly to empty and the Message Center shows Low Fuel. I start moving again, it jumps to the correct spot again (Unless it's full and it stays on empty).
I've been told it's the sending unit or corroded wiring and/or possibly the fuel pump itself. Not sure how to " trace" the wiring to see if corrosion is the issue or to test the sending unit before dropping $300 or the fuel pump before dropping close to $500.