For a gas, you usually want the root-mean-square speed for things like pressure and energy calculations—for N₂ at 300 K, it's about 517 m/s. The average is around 476 m/s, and the most probable is 422 m/s. But in my work with the gas pipe network in the hills, I actually watch for the average speed the most, because it tells me how fast the bulk of the gas is actually moving past a sensor, which matters for flow rates. I learned that from watching our irrigation channels—the fastest trickle doesn't tell you how much water reaches the orchard; the main body of the flow does.
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