Originally Posted by K3N R3D
What you said makes a lot sense intellectually, but consider this.
Since we all know RS is the performance version of standard Audi cars. And the CSL is the performance driven version of a standard M3. Why would BMW put 19" wheels on an M3 CSL if 18" wheels would perform better?
There is nothing wrong with 19" wheels but only if the car's suspension set-up was originally designed to run with said wheels. The reason why we are even having this debate is the fact that BMW decided to offer the M3 with two differing wheel size options and the reason is as goldminer said namely money, almost everyone will prefer the look of the 19" alloys on the car and are willing to for go a little of the balanced handling to have the looks. The kind of people who keep the standard rims are the kind who are only interested in the performance, the car could be made out of flower patterned Carbon Fibre and they wouldn't care less as long as it benefited the performance, same goes for the rims.
So what I am trying to say is you can only truly optimise a car for one wheel size, so this explains why the CSL was able to run 19" wheels and perform amazingly and yet in the M3 it always felt like a compromise.