Originally Posted by Coyot
Even if fine, forged 19s have the very same mass with "normal" 18s, this mass is spread around a circle of bigger radius than than the one of the 18s, causing more rotational inertia and thus impeding [more than 18s] acceleration and deceleration.
When it comes to brakes, 18s can house brake discs up to 360mm diameter (14 inches +). Really fine, sophisticated brakes of that diameter are capable of stopping anything, forever.
The question is why BMW had to make these M3-specific 18s so ugly.
If both rims were without
tires mounted I would agree with that statement. However, they do have tires mounted...
If the E46 wheel selection was any indication, the forged 19" wheels of the E92 M3 will be lighter
than the 18" cast wheels. This and the same weight or even lighter
19" tires (less sidewall but wider, probably slightly different compound) will make any difference in acceleration from the 18" wheels/tire combo insignificant. Done right (suspension designed for 19") and the difference in ride should be negligible. The difference in handling however, should be superior due to the stiffer sidewalls.
That's exactly the same concept used for the CSL: lighter wheels (although not forged but spun cast) and lighter tires than the OEM 18 wheel/tire combo.