The SR-71’s top speed is still classified.
Its just slightly below the speed of thought.
The top speed of the SR-71 was limited by the ability of certain components to handle the heat at MACH 3+ speeds. The engines were capable of more. It’s all about material science.
I remember seeing SR-71 fly-bys at the Reno Air Races. Truly awesome!
You can find some similar parts on X-15s that melted at mach 6. You have to use the sort of materials intended for RVs at that point. There was a project to test tips and fins made of hafnium diboride or something like similar for RV terminal guidance (SHARP?), that's the sort of materials needed.
30 years ago, while I was in college, my two room mates and I were all Military Science students and the university had what was (and still is) considered one of the best aerospace programs in the country.
One of our instructors was a former SR71 pilot. Heck of an instructor, too. One day outside of class, we asked him about the top IAS he saw on his SR71... He wouldn’t give us a number, but advised us the he watched as the leading edges of the wings turned neon blue with heat. He said he had always wanted to have a rear view camera so that he could watch the missles fired at him run out of fuel and fall back to earth... Before we could ask for an explanation of that statement, he held his finger in the air and then moved it across his lips in a “be quite” motion. He then smiled and walked away... I never herd him make a statement I felt was false, so I always wondered where he was when he outran the SAMS.
We had another instructor that was about 6’5” that flew F86 Sabres in Korea... He always laughed and said he didn’t get into the F86 - he walked up to it and put it on! :-)
Those were good days!!!