In my experience, sponsors/brands are looking for new ways to reach their demo aside from usual ad-buys etc, especially given the fact that ad sales are on the decline in just about all avenues (tv, radio, online, print)...so long as the venture will yield potential new business or at least reach their target demo, you'll always have an ear. What you have going for you is that the auto world and it's enthusiasts are some of the most brand loyal.

From my experience:
-K.I.S.S (keep it simple stupid)
-A simple one-sheet will suffice
-Have a plan B, should plan A bomb (or they may just wanna hear your other ideas)
-You can present a cross marketing concept with similar competing brands (if done right - pm me and i'll explain how i once did this)
-The more legwork you've already done, the easier for them to sign-on (for example, create mockups)
-Prove to them the amount of traffic your site receives or expects to receive
-Remember to relax during the pitch, what's the worst they can say, no?

Now if i gave you all my COOL ideas, i'd have to charge ya