I don't believe the headers or cold air kits do much for mpg. Face it, we drive vehicles that have the aerodynamics of a brick driving down the road. You may want to get it scanned and see if any codes are being stored that would lead the vehicle to be running rich. I think your truck should be getting in the neighborhood of 18-19 mpg, but with the tires and stuff I'm not really sure. Sorry I can't be of anymore help.