# SR-71 flew 2193 mph. How can this mathematically be Mach 3.3?

Virtually every time I read about the Lockheed SR-71 (including other threads on this forum), I see that its speed record is listed as approximately 2193 mph. The same blurb will invariably say this is "about Mach 3.3".

But when I search for how fast Mach 1 is, the number that almost always comes up is 767 mph.

Now 767 * 3.3 comes out to 2531, well above 2193. If I take 2193 and divide it by 767, I get Mach 2.85

I doubt I've stumbled on something new here. So what am I missing? How can 2193 mph be Mach 3.3 when 767 mph is Mach 1?

• Did you factor in temperature? The figure you found is likely for a standard temperature at sea level. The speed of sound changes with temperature, and it is much colder at the altitudes where the Blackbird flew. May 6 '20 at 20:23
• The graph in this answer provides a good overview. May 6 '20 at 20:29
• May 6 '20 at 20:36
• Ah, thanks all. I apologize for not picking better search terms. I should have been able to answer this on my own
– Joe
May 6 '20 at 20:41
• Don't apologize for that. Other may use the same search terms as yours and thus find this question that can help them. May 6 '20 at 21:00