Conus America Speed limit is currently 765 ish sea level on a 65 degree Fahrenheit, standard federal day. Lower as you go higher. Here at 1500 meters it is around 750. More than that and you'll attract all sorts of odd, irritated people. Particularly black jacketed people with 3 yellow letters on the back
Reasonable to believe enforcing a speed limit at altitude in any form would be difficult unless you are visible to the local airport radar which does have a speed calculation and display. Could be wrong but the local constable has a max speed limit on the radar they use, but it's much faster than what a typical car can go.