My gas tank only will fill up 8 galions every time I fill up and the gas gauge Dosent work but I alwase get 8 gallions of gas no matter how far I've driven and I don't know why

 

I would get a 5 gal. gas can filled and keep it with me, and see how long it takes to run out of gas. Another thing is you may have a bent/kink filler neck or hose stopping the air from coming out went gas is going in????