Going through lot of money on gas.
 

I own a 1991 Oldsmobile cutlass Calais it is the sedan, It has the 2.5L 4cylinder and auto trans. For a little while now it has been going through a lot of gas. My gas gauge is empty at half, and almost if not every day we have to fill it up anywhere from 20-40 bucks a day. I was wondering what would cause this?