I wonder how they could really tell that, since the speed of sound obviously isn't constant (faster with tailwind, slower with head etc), and the fact that sound can't actually travel in space.
That was the speed. You can hardly say "Oh, I went faster than this obscure number, I need another record!" And yea, he went to like 850 some MPH, so you can't say "The headwind made him go 80 Miles an hour faster"