Originally Posted by LarryJ

I've seen over 600 knots (690 mph). The guy I was flying with on my last trip said he's seen over 700 knots (805 mph) in the B777. Had 598 knots (687 mph) last week in the 737.


Ground speed can be supersonic but all that matters is the true airspeed (the speed of the aircraft relative to the airmass in which it is flying). True airspeed is usually in the 430kts to 480kts (495mph to 552mph) range.

At cruise altitude, though, we use mach number, not airspeed. The speed of sound (mach 1.0) varies with temperature. We'll usually fly in the M0.72-M0.85 range depending on conditions, economy flight plan vs. speed, and the airplane. The true airspeed that produces varies based on temperature.