This just started up a few fill ups ago. I will fill it up and it will register that its beyond empty. Then i drive it for awhile till its about 3/4 tank and it will register correctly as long as im driving with my foot on the gas. The second i take my foot off the gas it goes back to empty. Any ideas? Bad ground at the tank?

 

A similar problem just started with mine. When the tank is completely full the needle is erratic and constantly moving. After I burn through 3 or 4 gallons it calms down. I haven't noticed any correlation to the gas pedal.
After doing a little research, it sounds like the problem is caused by corrosion in the sending unit. The way I understand it there is a float in the tank that moves a contact along a variable resistor. When the tank is full the float/contact is in a position of low resistance, as the tank empties the resistance in the circuit increases. The fuel gauge is actually a measure of how much current is flowing. I think in my case at least, the portion of the resistor plate that corresponds to "Full" is probably corroded and affecting the current flow. That's as far as I've got. I don't know if the sending unit can be repaired/cleaned or if it has to be replaced.