[FtrPilot]

Was there any ground reference to the ground speed?

I am not sure this is an answer to your question. However, at cruising altitude (50,000 ft+) it is difficult to sense how fast you are flying. It is not like driving a car where 100MPH is fast and 60MPH is slow.

I can assure you, from experience, that flying 600 knots at 100 feet AGL takes 100% of your attention to keep from killing yourself.

The pilots have multiple readouts of airspeed in the cockpit which include:

indicated airspeed

true airspeed

ground speed

GPS provides ground speed. Prior to GPS, ground speed was provided by INS (inertial navigation system). Prior to INS, ground speed was provided by navigators using winds (direction and velocity) and a whiz wheel (picture below).

Personally, I have been mach 2 one time. Very uneventful.

[FtrPilot]

I should have included in my previous post:

Unfortunately, the mach 2 club is not as fun as the mile high club.

[CodeToad]

I was asking not of the instruments but of the sense of the speed.

I am private pilot and love the fact that I get to fly the airplane and not just punch a few buttons and wait my turn to punch a few more.