I just want to make it clear that changing the sidewalls isn't the work of a moment - for starters the texture has to be found and made, and positioned in the right place. I can't remember off the top of my head, but some cars default to the Michelin texture, and others to Cromo, whilst road cars don't even have the mapping on the car.
So, if you want a sidewall logo rendered that you haven't seen before, you might want to make one (or have one made). Ideally about 500*500 res (the more the merrier really), and with the new logo on all four 'slots' on the texture.
Hope that doesn't disheaten you all, as some of these skins are pretty cool. I just wish I had more time to render them for you. As it is I can only force time for a few; and besides, Dan is doing a very good job keeping up!