I have been watching this site lately(theautomobileforum.com) .. every hour or so there are several people joinining the site. I have watched membership go up from 5k to 8k in less than few weeks. But when you look at the posts there is hardly any activity going on. Also here is my site http://www.automobileforum.net .. feel free to review and if anything is wrong or needs to be changed. (I am getting a header/logo soon).
The site looks very nice, but with the membership do you ever get spam? Because could be bots I used to have that with my forum and I would say start posting yourself to get people to post.
Your forums are ok, you may need some hired posters on your site (see my sig) and have a feedback forum, that will let you know what your members think of it, and what you can improve on. Good Luck