I've noticed an increase in mpg of about 2-3% in the Mileage XS Saver programming with my truck which translates to about 20 miles per tank more than the stock program. I will say that the 87 Octane program has noticeable changes over the stock program. My truck has the 5.4L with the 3.73 rear end. I am running a K&N cold-air intake with an Airaid PowerAid throttle body spacer. In the Performance program I have trouble keeping my foot out of the accelerator because of the increases!