[Calgon]

Put the code under OnPlayerKeyStateChange, check if key = fire, perform a check of which weapon the player is using, use GetPlayerCameraPos, get the vehicle ID from the position (perform a loop of all vehicles in range of where the shot was at), use one of the pre-existing speed checking functions on the vehicle ID, send a textdraw with the speed etc.

Shady91 scripted something similar a while ago that he showed me ([ame]http://www.youtube.com/watch?v=K01H38bLVRc[/ame]) which may be released in his 'City Role Play' script.

I would have written the code for you, but I can't really be bothered.