Old Smokey,
T_0/T=1+(\gamma-1)/2*M^2.
I concur.
I've always wondered what the max Mach of the SR-71 was. The limitation was a CIT (Compressor Inlet Temperature) of 427C. I calculate that with ISA-50C, using the same equations Mach 4 was possible.
The flight manual (
http://www.sr-71.org/blackbird/manual/5/5-8.htm) lists Mach 3.2 as the design flight Mach number but in the book "The Untouchables", flight crew record seeing in excess of 3.5 on an operational sortie in exactly the circumstances I describe.
Richard Graham describes the problem with the SR-71 (on Pg 167 of "The SR-71 Revealed - The Inside Story) with different ISA devs.
I've seen ISA+21C and <ISA-20C so maybe one day, somewhere, it saw M~=4.
