They also monitor gear selection and throttle opening and maybe other things. 2400 RPM on the rear wheels of a car would be very fast don't know if the cruise control would even accept that. According to one online calculator, assuming 32" tire diameter it would be 228.5MPH !As far as I'm aware that's all they monitor? - but there's no real difference between RPM and MPH, you just need to supply pulses based on engine speed and let the cruise control think it's speed.
I guess my point is if you need a governor do it with a governor not a cruise control.