I mesurure speep by MPH and as far as the RPM'S funny you should ask i have been outside all day trying to get my tach to work i have tried all kinds of things only thing i can think of is tach is broke. It worked fine when I had the other moter on it. I have been cruiseing at 40mph by sound of the eng. it tops out right at 52 so i just back off by sound and run right at 40mph. I will be buying a new tach this weekend and see if that works and if it does i will let you know about speed and rpm's