I recently upgraded my M6 rims as the ACS rims were pretty outdated and it was time for a more modern look. They're 20x11 Vossen's on the rear, 20x9.5's on the front. I went up to a 305 30 20 tire on the back as opposed to 295's on my ACS's, which is the same size I run on my C63s coupe. I did have to run 15mm rear spacers and 12mm fronts to make these rims/tires clear the suspension. They drive perfectly, no vibrating or shimmying whatsoever, the ride is as good as stock if not better. However, I'm getting some pretty bad rubbing on my driver side rear on some bumps, and always severe rubbing when backing out of my driveway going left. If I backout going right, there's zero rubbing. The passenger side is perfectly fine, no rubbing regardless of which angle I reverse.

I took pictures of the exact spot where it rubs when I reverse with my C63s. I notice that the rear end doesn't sag which allows for plenty of fender clearance. The M6, not so much. The entire fender drops and lowers itself onto the tire. I get that these are 2 totally different cars, suspension setups, etc...however I have the same 2" fender clearance with the 305 tires on both cars, yet they both act very differently backing out of my driveway or on bumps.

I got my M6's alignment and suspension checked, everything looks perfect, no control arm, bearing/bushing wear. However the shop had no way of checking if the rear shock is good, but being that it only has 13.5k miles and these shocks usually last at least to 50-60k and often even longer than that, I don't think that could be the cause although I'm not ruling it out. Any ideas?

I made a quick video of the M6 backing out of my driveway, and pics of both the C63s and M6 at the same angle and point at which the M6 rubs going down my driveway but the C63 doesn't:

https://youtube.com/shorts/OwJBst4bU5Y?feature=share

 

I think you gonna rub on the C63 as well if you go for some aggressive corners. Those tires just look too wide. There's no clearance for the fenders at full suspension compression. I'm guessing it's just because of different wheelbases that the M6 compresses that much on your driveway and the C63 doesn't. Just look at the picture below. This is me going around the Nürburgring and you can see how the outside compresses. With your tires I bet you'd be rubbing badly.

 

Yeah you could be right, I think the solution might be to go with 295’s vs 305’s. I was running 295’s on my ACS 20”x10” rear wheels without any rubbing issues but these Vossens are 20”x11” so I thought 305 would be a better fit. But what I don’t get is why there’s not the same suspension compression on the passenger side when I go down the driveway going the opposite way. Wouldn’t both sides compress in a similar fashion? The passenger side is left with at least a 1” clearance gap no matter which angle I back out with so it never rubs.

As far as the C63S, thankfully I haven’t had any rubbing issues whatsoever but I haven’t tracked it or taken any really aggressive turns yet either.

 

For one thing, if nobody is sitting on the passenger side then that side is lighter, and in general things are not necessary 100% symmetric in a car. When I put spacers on my previous car I noticed that one wheel in the rear was further out than the other. Had the dealer realign the rear subframe to even it out more.

 

I finally got this all sorted a couple weeks ago. @superswiss was right, the 305’s were too wide for my M6 without some sort of fender rolling or suspension modification. I went down to 295/30/20 rears and down from 15mm to 12mm spacers. Apparently Michelin 4S 295’s are only .4” narrower than 305’s but also around .25” smaller in diameter, but that’s all the clearance I needed. No more rubbing or tire shredding! Ride quality is as good as OEM.