The weather changed very soon after I installed mine on the 4.0. The best way i can describe the mileage improvement in the short time that I had similar weather conditions-
you know how one time you get 18.3 mpg and the next time you get 19.5 mpg? The low/high range that a truck usually stays at? The 19.5 became the lowest mpg, with the occasional fill up that netted better.
I guess I would say a solid 1mpg.