problem with gas mileage

If you've checked for leaks and there isn't any I would think maybe the O2 sensor could be bad, but I would you should be showing a check engine light on the dash with an O2 sensor code . I would go to murrays or autozone and have them plug in a code reader for free and see if anything comes up.

the above answer os obvius, have your sensors changed, all 3 off them, all of your spark plugs + filter+ oil change. For further extra fuel milage put a cold air filter inatke on your car , should give you further 5-10% extra milage+5-8% extra horsepower. Add Millitec or similar friction reducing additive to your motor oil, should give you further 5-8% millage+ at least 5% more horsepower.. Hope this is useful, Regards from Reykjavik, Iceland