Average cruising speed is just an average. The plane isn't a locomotive. Its speed changes while in flight by large amounts. The plane is flying within mediums which move within mediums.
Average cruising speed is based on groundspeed, meaning that the system is using external reference based on ground coordinates in some manner. If those reference points are broadcasting Round Earth coordinates, then that speed was created using the Round Earth coordinate system.
This is all very unhelpful in attempting to map out a world which is not round.
Tom: You really know very little about how modern airplanes work - you might want to stop talking authoritatively about things you clearly don't understand.
*ALL* modern airliners fly on autopilot for 99% of the time. They MIGHT come off autopilot for a few minutes on takeoff and landing, in emergency situations, that kind of thing...but most of the time the autopilot flies the plane on all long distance routes.
The autopilot controls the speed to very high precision - so you're really talking nonsense here - just as you did when you claimed that newly designed airplane speeds are measured by test pilots rather than being designed into the airplane at the outset.
We have a couple of people here who are airline pilots - and at least one person who's an expert on flight simulation (which is all about this stuff).
Airliners fly at VERY constant speeds - and the distinction between air speed and ground speed - while important - is cross-checked in half a dozen ways by the various flight instruments.
What you're saying might have been true 40 years ago (maybe)...but for sure it's not true now.
So - fine, go ahead and talk authoritatively about the Flat Earth if you wish - but every single time you've said *anything* about how airplanes work or how they are operated, you make an idiot of yourself.