The SCT X4 does not come with a fuel-economy specific tune for the 6.7L Super Dutys. However, its tuning tends to provide a bump in fuel economy, assuming you drive conservatively. If you start mashing the throttle to feel the new power, you'll definitely see a drop. As long as you're easy with your foot, you can possibly see a 2-3mpg gain, depending on engine condition, fuel quality, climate, etc.