Ok, I get the sidewall ratio thing, but then why do all exotic supercars use thin sidewalls?
Anybody look at the side profiles (regargless of series) of the new Porsches, Ferrari, Lambo, etc. They are not even close to having the kind of sidewall that the stock 18" tires have on the M3.
Now the 19" sidewalls on the M3 look about right with the supercars, but the 18" don't, their sidewalls are definately taller then the supercars.
My thoughts on the 40 series on the 18" thus far are that once in the corner they stick well, but that initial transition into the corners, you can feel the sidewalls flex and that lessens the quick turn-in response.
16' M3 Mineral Gray, DCT, Adaptive M sus., 19" wheels
15' Audi S4
13' Audi TTRS (APR stage 1, MSS springs)
09' C6 Z06
08' M3 Interlagos Blue: 6sp, Tech. & Premium Package