Mach"s constant- 760 MPH at ground- sea level. this decreases with altitude- the Famous French Fighter, the Mirage III flew demonstration runs with a ( valve cutoff) of Mach 2- at precisely l.350...
The amount of air in the tire stays the same.The outside pressure drops about 3psi or 0,2 bar, when you are at about 2000 meters or 6000 feet. So you would expect that the pressure you measure is...