Modified is correct, that forums are notoriously difficult to maintain active.

A generic Car Forum is unlikely to develop much of a following, which means you'll be counting your weekly postcount on the fingers of one hand, unless you succeed in building that following. There are several more specific niches within that generic heading, however, that are much more likely to build the sense of community that will help keep a forum alive and growing.

I would suggest sub-boards, such as Car Restoration, Car Shows, Hard-to-find car parts, and the like. More specialized groups such as those may be more active, and inspire more contributor loyalty.

Good luck!

Edit: Assuming you have the bandwidth available, encouraging photos of restoration projects and shows, will also help build that following, and may help you in web visibility. Just make sure they're uploading photos by linking to their ImageShack or Photobucket accounts.

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