Gas Gauge Questions

[Y2K_MustangVert]

I have been noticing that my the needle on my gas gauge hasn't been functioning correctly. When I go and fill up the car it reads F, but after driving only 80 miles on the highway it now reads between 1/2 and 3/4 of a tank. Normally I can get 80 miles out of it before it even drops from "F". I know something is wrong, but do you guys(and girls) think that it is the sending unit in the tank, or the gauge itself? Any help would be greatly appreciated.