Just a heads up, this should be posted in the content or game support sub-forum. This post will be moved by a moderator to the correct forum.
Anyways,
Go to RoRconfig, click the 'advanced' tab.
We're hoping you're using above 0.4.0.5 here.
Uncheck both the boxes- 'Disable Inter Vehicle Collisions', and 'Disable Intra-Vehicle Collisons'. This might lower frame rate on weaker computers, like a celeron, core 2 or atom, but my dual core, 8 year old i5, intergrated, 64mb graphics laptop can run with collisions very happily, with 3/4 cars at an acceptable frame rate.
Or, if you're using an older game version, go to the .truck file and remove the semicolon ';' that you added to 'contacters' and/or 'Submeshes', if you were around in the .39 and below era. Or if those are gone, redownload the car.
Make sure you're using vehicles that actually have collision, some do, and some don't. IIRC the w460 pack has no collison with other vehicles.