More proof that you want the 19 inch rims for best performance.
Not to mention the 19 rims look infinitely better than the 18 inches that look too small and SUV-ish on the M3:
"When new, our M3 gripped the skid pad at 0.90g, while in wrap-up testing it generated 0.94g. The slalom revealed the same behavior pattern, as it recorded 70 mph when new, and then went 70.4 mph at the end. All of these numbers, however, are slower than the results when we fit the car with the optional 19-inch wheels."
""A great sounding, responsive, high-revving, naturally aspirated engine is part of the DNA of a thoroughbred sports car. No two ways about it."
- Lamborghini on turbocharging