I guess you do not understand the difference between Ground speed and Indicated air speed. If you have a look at a GPS when at cruise you will find the two are not the same. When above around 30000 ft most planes use MAC and fly around .78. Planes use differential air pressure to measure air speed, and the air gets thinner as you climb so the indicated air speed gets less. around 2% per 1000 ft. So a plane doing 370 kts IAS at 40000 ft is actually doing around 666 kts ground speed, not taking into acount wind speed and direction. I think you need to do some reading. And the reason they use MAC is because as the air gets thinner the speed of sound gets lower, little red striped bar on speed indicator, you need to be below this if you dont want your plane to fall apart.
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