Why cars in multiplayer have weird motions like they are lagging or something? I also noticed that my car "lags" too on replays. I am talking about this: https://youtu.be/pbklUzptfTk (0:45). Is it because i use demo version of LFS?

Demo has the exactly same netcode as licenced version. The only reason is connection quality - you can see your ping in connections list (press N until it shows up),high or variating ping will cause very unstable cars movement. Also check where the server is located - if you're not in European server,your ping will be increased by default already.

My ping is 49-51 . I always sort servers by ping and join the lower ping servers. Anyways, thanks for your reply.

Also, I checked my ping in connections list and it says that my UDP/TCP is 40-60.

Yeah, that ping is not high enough to cause any weirdness. Is your fps stable? That kind of stutter may be also due to somewhat lower PC spec. Currently, LFS physics runs at 100Hz and if you have a screen refresh rate that is a non-integer multiple of it, that is normal.

Are you saying that video (https://youtu.be/pbklUzptfTk) runs at 5 fps or something? LFS server just sends too few packets to make cars drive smoothly . Do not reply guys, I don't need help anymore.

Car update rate depends on the server and it's increased to 12/s since the new upates. LFS has the interpolation or prediction of car pos and speed which does the small steps inbetween and it does it quite well, for the little data rate that it requires between server/client. What you complain about is due to a missmatch between LFS physics and screen refresh rate, so some frames must be skipped and this causes a "jerking". Set fps limit to 100 and it will be much better, ofc you PC needs to be able to keep that fps stable.