Are there any "spinning wheel" rims that will work on a Mercedes 450 SL? I've only seen them in sizes 20" and up. I think they'd look cool on a benz. Is this possible? and are there other sizes of these kind?

It's possible! I Think they look great check w/ the various manufacturers for your size, I'm sure they got it. 5x112mm bolt pattern.

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2003 E55 AMG
2007 FJ Cruiser

http://truspinners.com/

i found this link that lets you add spinners to your existing wheels for a much lower price. it looks cool to me. i'm new to the whole spinning thing, never knew about it until a couple weeks ago when a seen a suv with them for the first time.