This theory has bugged me ever since my first physics class on the subject. If this (http://en.wikipedia.org/wiki/Kinetic_theory) is true, it leads me to a few weird conclusions.
Opening the rear window in a pickup truck at 10 m/s doesn't empty the cabin of all its air. This means that gas molecules can catch up with us from behind so we can conclude that gases move much faster than 10 m/s at normal temperatures.
Then, shouldn't gases mix almost instantaneously? For example, shouldn't a fart's smell reach us faster than a car on an highway and then dissapear almost immediately into such a large area that the concentration is too low for our noses to detect?