I just replace my fuel sending unit last weekend. I think they work off of some type of resistor (same kind they use in the slot car track controllers), eventually these get worn and can no longer read, my float worked fine, but stayed full until a 1/4 tank and then dropped, didnt have much time to get to a gas station! I got a unit cheap off ebay, brand new ford factory part, works great now! I installed a 255 pump to make it worth my while.
Anthony