We know that the flash doesn't tire out, but he's still running. When other people run they tend to sweat, does the Flash do this? Has the comics ever show him sweating after any sort of run?
Which version of the Flash are you talking about?
In the comics from the 70s, I've only seen him sweat when it is really hot out (and he's not running), nervous, or when he's sick with a fever.
I seem to recall him sweating from over exertion but in the race with Superman, (when he is presumably going all out) he is not shown sweating.