Jazzy.....In simple terms computing the speed of sound @ different altitudes ....hence different temperatures........is the same for computing Mach Number because they are in direct proportion to each other......flying @ Mach .75 is flying @ 3/4 the speed of sound at a particular altitude........In a perfect world without any wind to contend with......@ .75 Mach at 33,000 ft would equal a higher ground speed than .75 Mach @ 41,000 ft........because of the change in temperature( Decreasing).....its much simpler than most would think........Bill......