MM, mine is 59 MPH in my Winnebago. I thru the years owning this coach kept track of fuel mileage and I get the best mileage at 59 MPH. My tach is at 1900 RPM's slightly lugging, which causes the engine to run quieter (engine not roaring or making a loud sound). And for some strange reason the coach rides better and, I don't get the bumpy ride in my coach as when I'm driving 65 MPH. That's how I determine my sweet spot in my RV. This is one of the more interesting questions posted on RV.Net! Thanks Mark, Outstanding effort!! I'm still trying to find my sweet spot when golfing? That's another story, my golf ball spends more time in the trees than the squirrels. My golfing does get better golfing in the desert!