Agressive tread pattern can kill mileage. Also, even in stock tires, a few more pounds of air can give dramatic increase in mileage due to smaller contact patch and less rolling resistance.

If you have illuminated CEL, the engine goes into limp home mode, which is to retard the timing, and set rich mixture in an attemt to not cause detonation, or high EGT from the lean condition it believes you are having.

John C.