If you don't change your driving stlye, the power button does nothing...milage won't change. It's simply a different tranny program, that holds a gear till almost redline, when the throttle is held at 75% WOT or better...on a hill, again, with your foot in it a bit, it will kick down a gear a bit sooner. Since our thrashy ol Isuzu 6 runns out of breath long before redline, it really only helps to about 4500 rpm....the last couple thousand rpm is all clatter, and agricultural noise. My power button is always down, and I'm getting 18-20mpg combined. I rarely (almost never) go over 3/4 throttle.