What kind of spark plugs did you install??? I would clean the Maf sensor and if possible hook up a scanner that you can watch how the prercat 02 sensor are working, if they are lazy you will get poor mileage. The rear 02 tells the pcm how the cat and precat sensors are working by detecting carbon monoxide and hydrcarbons.
If you remove the precat sensors and the cat is plugged the vehicle will run much better, you could also disconnect the precat sensor and see if that makes a difference in mileage as the pcm will set to default fuel air mixture.