Has anyone noticed that their MPG numbers go dramatically downward as you reach the end of the tank?

I don't have enough evidence to support my theory yet, but it appears that everything is fine and dandy with MPG until the gas gauge drops below the 1/2 mark. Then, no matter what I do to increase the MPG figure from that point to the 1/4 mark and lower, I keep *losing* MPG instead.

If this really is true, then that means inorder to maintain MPG numbers, I will need to fill up at 1/2 tank, which doesn't make a lot of sense to me. Anyone else's input?