This thread started talking about knots - nautical miles per hour. One knot = 1,151 mph or 1,852 kmh.
One nautical mile is simply one arc minute at the Equator. And the km was defined as 1/40,000 of the circumference of the Earth. And there goes 60 arc minutes on one degree of which there are 360 around the globe. So one nautical mile is exactly 40,000 / (360 * 60) = 1.8518518 km. What a mile is, that's something which the Americans must explain. At least the Englishmen are now slowly going metric, inch by inch.
And all this is groundspeed based on GPS calculations. It can be airspeed and anything from plus or minus 300 kmh - that's how strong winds can be up there. My fastest ever was between Zurich and Madrid on an A320, 1164 kmh (628 kn, 723 mph). Had it been airspeed, then at our altitude 31,000 ft. we would have been supersonic. Speed of sound is not constant, but decreases slightly as air density decreases. Sure the Swissair accountant smiled when he saw the fuel bill for that flight. Back again a couple of days later, also on an A320, I never noticed more than 750 kmh. Sure there was headwind, but not nearly as much as the former tailwind.
Best regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs