We've tried this with a forum for selling things, and posting about upcoming sales. Sure occassional posts were made - once a month, maybe...but it just wasn't worth the clutter of having a forum for it. So far, there haven't been any threads about cars even started - so there's no need for a forum.
To be blunt about it...it's not going to happen:
- The need is not there
- The clutter of an additional forum (while we're trying to reduce the number of forums and still maintain organization) is not feasible
- Cars in particular are not really something this site has focused much discussion on...or really anything like it.