The point that you guys seem to be missing is that when the tank is full, and the needle is pegged, it still has farther to go. If you were to remove the stopping pin at the full mark, the needle would go beyond it, and you would see start to drop right away, as gas was used. It's not that the float design is such that the sender continues to think the tank is full until you use up a gallon or two so that the float moves. The float moves right away.
I learned this in an Automotive Electrical Systems class I took at the local cc when I was preparing for my ASE A6 cert. They had us measure the resistance from the sender. We filled the tank full, and then used the fuel pump to drain out fuel. The resistance started to change before we even had a quart pumped out.
Interestingly enough, I am currently trying to advise a gentleman on F150online as to how to set up his gas gauge so that it reads correctly. He removed the needle from the gauge so that he could paint it, when the tank was full. When he pulled the needle off, the shaft it sits on rotated slightly, without him knowing it, so that when he put the needle back on, it read incorrectly. He said he knew right away that something was wrong because the gas gauge started to go down right away, rather than not moving for the usual first 20 miles or so. When the needle got to the empty mark, he filled it, and the 19 gallon tank only took 12 gallons.
Anyway, food for thought.
Take care,
~Chris