If its filling a gap in the market, it should grow. I got mine a domain name, splashed a few links around, and before I knew it, 'Iveco daily' on google brought me up number #3 or 4 in the listings (just after Iveco themselves and a couple of big dealers) and add the word forum gets me #1. Its two years old and has 7600 members.
All I would suggest is less subforums is more. Too many forums is a maintenance nightmare. Look at this place...four forums and its great.