I have the same setup 1997 Explorer 215, Evinrude 150. At 4200 RPM I am running in the Long Island sound 40MPH. I have had it up to 45MPH but that was about 4800RPM. I heard cruise is around 3600 and 4500 is when you need power. I have seen these engine run up to 6500 RPM. I am a good 4 stroke mechanic and have a race car that I work on all the time. I am not sure how these engines run. Maybe someone can chime in?