FYI my stock tires where the 18" wheels with 285/65's on them, the grearing is the 3.73's I am pretty certain. The size difference in tire I am running now is about 3/4" taller and 1.5 inches wider. I didn't think this would drop my mileage this much.. I am now just at 14.0 mpg and seems to still be dropping. Any ideas on what besides the tire change could be causing this? Should I go with a CAI system and dual the exhaust? i.e. better breathing=better performance, maybe improve mileage a bit.

 

Your larger tires are probably 40% heavier which impacts performance more than the size does.