From what I've experienced with my SCT and what I've heard from other people there is no significant mileage gains with the programmers for these trucks. The 6.4 is a efficient engine by itself, but when paired up the the new exhaust systems the mileage is horrible for most people. But if you want to spend the money now you can go with a programmer that allows you to remove the DPF on the exhaust, 2 - 4+ mpg increase while empty and towing.