all vehicle physics has to calculated server side so the time it takes your system to send the data to the server and the server to send it back again results in "lag" responsiveness ... unfortunately the only way to fix this is to re-write all the vehicle code nave to the UDK making the vehicle physics all client side on only sending position data to the server (this is alot of work) but also but a lot more strain on the client's network / internet (allot more unnecessary data)
with the current UDK set up the faster the vehicle is, the worse the lag seemingly becomes.
pros vs cons