Crossword Genius

__ number (SST stat) (4)

Ross

I believe the answer is:

mach

(Other definitions for mach that I've seen before include "-- Number relates to speeds" , "Ratio used to measure high speeds" , "Ratio of a body's speed to the speed of sound" , "The speed of sound is ... one" , "Austrian physicist, gave name to speed no." .)

I've seen this clue in the Newsday.
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