oh HELL... moneybags is looking at rims now... this thread has some potential.   

1st thing you have to know is bolt pattern... for BMW thats going to be 5 - 120 in other words 5 lugs (120 mm in diameter?)

2nd I'd think about what size you want.  I think the M6 stock wheels are 19"s.  Do you want to go bigger (I'd assume so, but there are pros and cons to that decision.)

• 20 inch tires are typically cheaper than 19" tires (example my tires (20s) were cheaper than Sean's (19s)- his are wider 9.5/10.5 but mine were quite a bit cheaper)
• 19" rims are lighter and generally considered to be more performance oriented.  Once you get into the 20-22" range you're really getting more show than you are go

Now after you've decided on the rim size you gotta consider rim width.  Making sure the rim isn't so wide/tall that your tires rub is the key.  thats where offset comes in to play. 

Zero Offset  - The plane of the hub mounting surface is even with the centerline of the wheel.
Positive Offset - The plane of the hub mounting surface is shifted from the centerline toward the front or outside of the wheel.
Negative Offset - The plane of the hub mounting surface is toward the back or brake side of the wheel's centerline.

stock offsets
Front 8.5J 19" Offset +12
Rear 9.5J 19" Offset  +17

generally speaking the lower your offset the more agressively your wheel will look.  For example the factory offset on the M wheel is probably over 50.  After market wheels, like the ones I bought from wyeth are going to be more like 35-45.  this will push the rim further out into the wheel well giving it that flush look that everyone loves.   I'd have to do some searching to find what offsets you'd need for the M6, and it would heavily depend on your wheel width.  Obviously a 8" wheel with a +25 offset wouldn't sit out as far as a 9" wheel with the same offset.  So post up some pics of rims you like and we can help you with the fitment.

Wheel fitment comes into play through several distinct measurements.

The first is bolt pattern.  As Brandon mentioned, the BMW uses a 5x120 pattern.  That means there are 5 lugs, and if you took the top bolt in the start pattern, its 120 mm away from the horizontal line that would cross the very bottom of the bottom 2 bolts.  Use this to visualize.  The vertical line would be 120 mm for BMW wheels.


Centerbore.  This is the machined hole that sits is in the middle of your bolts.  OE designed wheels have a specific centerbore size that perfectly futs over the hub, centering the wheel on the bolts.  I don't know what BMW uses, but some wheels are BMW specific so will have the OE size centerbore.  Other are designed to be generic, so they have an oversized centerbore.  Then you'll need hubrings, which most wheel shops will be able to provide.  They are machined rings where the inner diameter is the size of your hub, the outer diameter matches the centerbore of your wheel.  Without these, your wheels will be resting on the wheel studs themselves, which makes it a little tougher to properly seat the lugs and can put a bit more lateral stress on your studs.

Wheel width.  Pretty obvious, the width of the wheel.

Offset, now this is all important and completely tied into the wheel width.  As Brandon stated, you can have 0, positive or negative offset.  If you take your wheel and divide it down the middle where the hub face is, if that hub face is closer to the front of the wheel, you have positive offset.  If its exactly in the center, you have 0 offset, if its closer to the rear of the wheel, you have negative offset.



So we'll play with a couple numbers.  Lets say you have an 8" wide wheel.  If you have 0 offset, there is 4" of wheel before the hub face and 4" behind.  If you have a +25 offset, you'll have ~3" from the face of the hub to the front of the wheel and ~5" from the hub to the rear.

This means that when your wheel width stays the same and your offset decreases, the wheels move further out towards the fenders.

The problem is offset is completely dependent on wheel width.  If your wheel width increases, but the offset isn't changed, that extra width is added evenly before and after the hub.  So a 10" +25 wheel would have 1 extra inch of wheel sticking into the fender well, and an extra inch further out towards the fenders.

How do you know what fits?  Well, you can either take measurements and figure out how much fender clearance you have.  But the reality is that many wheel manufacturers who design wheels for your car will have some specific sizes laready made.  Best place is to see what other people are running, and how the fitment is.  Shops that specialize in BMWs and have experience with fitments are pretty good at giving you an idea of what fits.  Unfortunately, no perfect way other than measuring, trial and error and seeing what other people have done!

I would pick a German brand, BBS, and choose a DTM touring car style, and would choose the lightest wheel combo - whatever the design, I'm sure that with that much racing heritage, it can't be that bad.

Holy crap this is complicated.  Uhhhh, anyone know where I can just find a simple shop?

I have poked around http://www.rims-n-tires.com/ before. I found it to be pretty helpful....

did you try tirerack, just to get an idea?

I can NOT remember the name of the site I was playing with, it allowed you to put on different rims, lower your car, try different backdrops, seems like you could even tint your windows.  Wish I could recall the site, but here's one I made a year or two ago, just as an example:

OK, I'm off to TireRack.  I've been doing a bit of looking around at wheels.  Found a few I like.  Still, I have a bad feeling Im going to end up going "Yeah THAT one"  And then it will be the wrong size.  My favorites thus far are the SDS Wheel but to be honest, it would look bad on the M6.  Sooo, the one I like now is the VSU by Vellano:
http://vellanowheels.com/collection/VSU/vsu.htm
I seriously hate black wheels but this two tone thing has me very intrigued.  I'm 100% certain i'll change my mind.  Till then, still looking.
P.S. so, do I have to buy new tires too?