What Tubby said, times 10! There are so many other car forums out there, why not join one of them if you like chatting to people and discussing car things? What's going to make people want to join your forum rather than one of the others that's out there already?
I've got a website in the automotive niche and visit car forums quite regularly, and I've nothing but bad news for you ... general car forums don't work; they only attract spam posters like magnets and never form a real community, which is what a forum needs - a core community of posters that engage each other regularly. More focused car forums do better because they tend to get the fans in but they are not hyperactive communities.
You need to identify your core posters in the real world - like Tubby suggested, get some of your buddies that not just know about cars but are car fanatics to be your core community. Forget about hiring posters, I've seen some car forums do that - 10c a post for 25 posts and all they get is 25 crappy posts by people who don't know anything about cars and disappear after 25 posts. That doesn't build a community and it also sets the tone for the forum.
My advice would be to identify your friends and other people in your community who are nuts about their cars ask they what they'd say about joining your forum and discussing their cars, their dream cars, the cars they hate, etc. Keep it on topic, which means drop the "General Chit Chat" forum - that's just a place for spammer's post their crap that has absolutely nothing to do with cars. Ans nip the crap posts in the butt - those "nice car, I like the color [hyperlink here]" posts that don't engage anyone.
It's a long hard road ahead if you really want to make a success of it. Sorry if this sounds like doom and gloom and good luck with it.
