Okay, I've contemplated this question for a while and have finally at least made an attempt to answer it for myself. I recently installed the 18X9 ET 34 AMG Monoblocks with the 245 X 40 X 18 Michelin Pilot Sport tires on the front of my 500E. They Fit! I did however have to roll the fenders for clearance. The right tire was scrapping the fender just a tad so I rolled the fenders to full flat. The 245 X 40 is a bit small for a 9 inch wheel so I will be going up to a size 255 X 35 X 18 Michelin Pilot sport. To tell you the truth, I'd really like to go 265 X 35 X 18, but fear that that may be pushing it a bit.
Ultimately I think I will just try to find what is the largest front size and go that size all the way around for rotational purposes. (Although I am seriously thinking about going with the 285 X 35 X 18 Pilot in the rear just to give the car a bigger fatter rounder prettier butt
