Ernst —, Austrian physicist who devised a system of speed measurement (4)
I believe the answer is:
mach
(Other definitions for mach that I've seen before include "Austrian physicist, gave name to speed no." , "Ratio of a body's speed to the speed of sound" , "Ratio of the speed of a moving body to the speed of sound" , "Ratio used to measure high speeds" , "-- Number relates to speeds" .)