I was just having a look at Singapore airlines flight SIA211 at flightradar24.com and at one stage the plane was moving at a speed of 771mph at 38000 feet. 771mph!!!!!!
. Ground speed, though. The plane is Boeing777 and their rated max cruise speed is 590mph. Must be a hell of a tail wind at that height. A Qantas flight, QFA32, from Heathrow, was doing 750mph at 39000.