I am making a real time tank strategy and my problem is that the tanks from
diferent users are not synchronised (meaning tank1 in user2's instance of the
game is not in the same position as tank1 in user1's instance) if i make the
velocity and rotation of the tanks as shared objects. If i use position of the
tanks as shared object then the tanks "jump" around when they move faster.

Can anyone help?

Synchronization is a big issue on real time multiplayer games.
There is an old article that might be of help four you on gamasutra: =

http://www.gamasutra.com/features/19970919/aronson_01.htm (you must =

register to read it)
You can also seach the articles on gamedev.net about multiplayer games =

(http://www.gamedev.net/reference/list.asp?categoryid=3D30)