Used to fly F-104G... 1965-1968 - Belgian Air Force, EBBE 1st Wing, 349/350th Squadron.
Often I was asked "what is your max speed", IAS, (TAS) or Mach...?
We gave "numbers" to be 2.3 Mach... but really it was not.
The real limit was a temperature
The correct answer is -
Tt2 - Engine inlet temperature (J-79).
At Mach 2.3, the rise was about 150º...
At FL 600 or so, Mach 2.2, typical SAT was -60º, so Tt2 was about +90º.
Just a little red light that flashed next to the IAS indicator.
If I remember well, limit was 100ºC ram air temperature that triggered the light.
xxx
My turn to be curious... Did you have similar engine concerns in Concorde...?
Mach 2... FL 590...?
xxx
Happy contrails