Mark.....MTThomps.....You guys are correct as my understanding goes....Being an aircraft mech I see and talk with pilots most everyday about this topic in some form....most of all todays A/S indicators are electric/CRT/LCD...deriving its info either in analog form or digital form from a ADC/CADC (Air Data computer) from a TAT (Total Air temp) probe on the Fuselage in the fwd nose area depending on Manufacturer.....one of its main purposes in life(Amongst others) is to provide direct info to compute Mach Number....keep in mind "Density altitude" computation is temp/pressure compensated....where as Mach number is temp dependant only.......Above 20,000 ft as a rough measure as a plane increases altitude by 1000ft indicated airpeed drops by 2 %......so @ 21,000 if your at 350k indicated @ 37,000 ft you would be roughly 245k indicated.......also keeping in mind the lapse rate( rate @ which temp decreases) is roughly 3* drop per 1000 ft increase in altitude.....which influences density altitude......decreasing as well.....Bill.......