Check to see if all your vacume lines and hoses are properly connected and not pinched or leaking. A clogged pcv valve or fuel filter can also cause poor fuel mileage along with numerous other things. I'd recommend a tune up, along with what i mentioned above. I, once, accidentally left the bottom line( the line that goes into the firewall) to my charcoal canister disconnected after changing my fuel filter, and it caused chitty gas mileage.