Hola Che -
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The TAS of Mach 1 is 661 knots at 15º C static air temperature.
It varies with temperature ONLY (altitude does NOT change anything).
If temperature increases, say 25º C, Mach 1 will be 671 knots TAS...
If temperature decreases, -56º C SAT, Mach 1 will be 574 Knots...
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That is the best I can read on my Jepps CR-3 slide rule...
So, Mach .85 with -56º C (standard temp at FL 360) is 488 knots TAS.
If temperature increases to -46º C (still at FL 360, Mach .85), it is 498 knots...
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Happy contrails