When using the speed command on a vehicle it seems to work fine.
However, if you use it on a projectile for example, it shows a constant incorrect value.
At first I thought the ballistics were faked (Constant speed with faked drop), but as it turns out, it's the speed command.
Instead of the actual speed, it seems to return the initial speed of the projectile. {F21898}
Download test mission and fire the tank.
Note that the value from the speed command remains constant.
The second number shows the actual speed (velocity magnitude).
Also, note the exponentially increasing speed of the 120mm projectile from the Slammer tank, described here http://feedback.arma3.com/view.php?id=13936
For now the workaround would be to use round's velocity vector instead and push it through BIS_fnc_magnitude function to obtain numerical value.