What is the fastest flying commercial aircraft?

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Fastest Commercial Aircraft

(for one of the people who answered altready the mach number does not change with air density, but with temperature.) As you climb, the air gets cooler (more dense), but as there is less overall air, the net is the air is less dense. While the less dense air decreases the avaiable power to the engines, the side effect is also that (1) there is less drag and (2) the fuel/ air ratio for combustion is less. So, while there is less power available at high altitudes, less power is required to fly a speed and less drag is there so the fuel burn is significantly less. In terms of speed, the aircraft can fly a "faster speed" the higher it goes as the power it looses it less than the drag it looses with less dense air, but it is so small it most cases that it is not the reason we fly higher. I just ran some numbers for you and between the surface and 18,000ft the speed capabilities are different but above thay the difference in speed is pretty small. For reasons that don't need great explanation for this quesiton, the "indicated airspeed" will have a larger difference to the "ground speed" due to air density, but the ground speed at 35,000ft will closely match the ground speed at 20,000ft. Secondary reasons, as someone else stated is that most weather is located 35,000ft and below so the higher you fly the better chance you can get above a thunderstorm and not have to go around it enroute. In addition, the higher your aircraft can go the less traffic you will encounter so the less potential enroute delays. So... in an way too long answer to your excellent question, aircraft fly higher for better fuel burn and secondarily weather and ATC delays.

Presently, average commercial planes fly at 550 to 600 m.p.h., and with the official retirement of the Concorde in 2003, the Boeing 747 is narrowly the fastest commercial airliner, capable of speeds up to 720 m.p.h.

fastest flying commercial aircraft

Also depends on which way the wind is blowing. If you have a tail wind, the ground speed of any aircraft increases...