# The SR-71 flew at 2193 mph. Mathematically, how can this be Mach 3.3?

Virtually every time I read about the Lockheed SR-71 Blackbird (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 $$3.3$$". However, when I search for how fast Mach $$1$$ is, the number that almost always comes up is $$767$$ mph. Now, $$767 \cdot 3.3 \approx 2531$$, which is 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 $$1$$?

• Commented May 6, 2020 at 20:36
• great question, thanks @Joe - whilst I knew that mach is meant to vary depending on air conditions, I did not know the details - and now we have all the details! Commented May 8, 2020 at 14:21
• Well you see, multiplication works differently at high altitude.. Commented May 8, 2020 at 19:40
• Because Mach number as a function of temperature, not physical speed Commented Feb 27, 2023 at 17:48