I would say that generally around 120 MPH TAS is the sweet spot for my 125 HP Series 5 (Continental IO-240) . I can go a tad faster with higher power settings but everything is smoother at the 120 MPH setting which is about 65 % power. Others will chime in here ,but I suspect you will find 120 MPH to be the majic number at least on the well powered later model Kitfoxes. Bruce N199CL