MotoBabble launched a few weeks ago. It's a full functioning auto social network with some work still needing to be done. It is all in house coding that we have worked on over the past several months. MotoBabble looks to take all of the functions of car forums and put them into one website where the user has a uniform experience.
MotoBabble allows the user to create albums and upload unlimited photos of their vehicle, create mod lists and set ups, invite friends and integrate with other social media sites, ask questions, answer questions, and vote on other answers, and start discussions in specific vehicle communities.
At the moment we are using grassroots efforts to spread the word and continue to implement new features on the site. I'd love to hear your thoughts and/or suggestions.
Link: http://motobabble.com
If you can figure out a way to get these forums to migrate to your platform (yes, make it a platform), then I think you're on to something.
The other option is to just create the communities from the ground up and let the users fill in the gaps in knowledge.
People tend to have particular tastes when it comes to cars, so I think it's good to form communities around makes and models early on, or more general categories like hotrods or european sports cars.
Of course, I usually only go to car forums when looking up how to do a repair or something on my own car. It's a classic Saab, so the forums are simply the best place to get information on repairs.