Hey there. I have some questions about tweaking in general and mods from lfs-tweak.com ..

So basically I download a car tweak, like one of these

I start the game, start the .exe file, and press Activate. Usually some sound plays but nothing changes ingame. Ive tried several different tweakers and I dont understand how to get it to work..

Another issue I'm having is when using the LFS 0.6b Pro Tweak 1.4.. Whenever I launch it it displays an error about "InvalidArgument=Value 146 is invalid for SelectedIndex"

Any help is greatly appreciated, I've been trying to get this to work for hours and I cant seem to find any solution..

Tweak isn't really allowed here on forum, and I doubt someone here have knowledge to help you.
You are better to ask on that page you downloaded this

Oh, thought it was allowed under Unofficla addons :P I'll try else where then