Originally Posted by b1aze
You are 100% correct. Assuming one does not change the overall rolling diameter, you will see no performance increase from going with a 17" wheel over 18" wheel, assuming weights don't differ drastically. Rotating mass is rotating mass. Its all still centered around the axle.
I like the BMW performance suspension also, i plan on getting the setup soon.
Cool -- I'm leaning towards 18s. I usually don't care too much about cosmetics -- function over form, I say -- but in this instance, I do prefer the look of 18" rims and I think the ride quality will be OK once I ditch the runflats.
Odd reality: the ride quality on my 335i Coupe with M-Sport (i.e. sports suspension and low profile 18" performance runflats) is significantly better than my wife's E30 328i with the standard suspension and higher profile luxury runflats. Go figure!