The absolute simplest way to limit speed on the pie5's is using the bluetooth and phone app, when you drive it you have speedo and more importantly rpm, when your max speed is reached, note rpm, go to settings in app and set that rpm as max, remember to store data to pie.
After that you have full power until max rpm is reached, very handy, btw you can also limit accelleration to i.e. 50% so it doesnt kick off at start but genly brings you up to (max) speed.