Comment by ufmace

Well as long as we're getting technical, Mach number is a unitless aerodynamics term, not a speed. As such, it is only valid in the exact medium that a craft is traveling in and relative to that medium, so here air at the exact temperature and pressure the aircraft is flying at. There is no valid general conversion between speed and Mach number, and things like Mach number relative to the ground is nonsensical.