Did you do any carb tuning after the dual exhaust? Do you have a wide band O2?
When you compared mileage, was the pre-dual exhaust from the summer and the dual done during the winter? What I'm getting at is they change the fuel composition fro winter time, and its different that summer fuel and in the testing we conducted the winter fuel gets worse mileage in an apple for apples testing environment over summer fuel. Your regional seasonal change will dictate when that happens. Also, the weather alone will change your mileage, wind, puddles, snow, mud etc will effect your mileage if your conducting testing on the same road.
