Speed through air versus Speed across the ground
Scarbagjack, the speed you are seeing is the ground speed and is quite normal when in a jet stream. I have regularly flown from Zurich to Bangkok at 38,000 feet with an indicated speed of 1111 kph, quite exhilarating.
If you tried to measure the speed through the air it would probably be 250 kph (that's not a fact) as it is measured against local atmospheric air pressure and hence we end up with actual speed which is measured in Mach, and there your 744 would be rolling on at M0.84.