(Source: A BMW enthusiast message board)"...wheel/tire weight makes a HUGE difference in acceleration and braking... A typical 15" E36 wheel/tire package weighs in at about 36-37lbs. 15lbs for the 15x7 wheel and 22lbs for the tire. I even noticed a difference when I sold my 16.5lb BBS RK's with 20lb Pilot Sports, when I went back to stock. Even though there is only a few lbs of weight difference, the sheer size of 18's make them more difficult to get moving. I had a 55lb 18" wheel/tire package and it RUINED the feel of my car. People with heavy 18's/19's make me laugh...what a waste of money to make the car perform worse."
"Yesterday I decided to take my 18 inch Sport Edition Teknos off for the winter and replaced them with my factory 15s....I was amazed with how much faster my car is with the stockers! There is a definate noticible difference in my 325ic's acceleration, cornering, and braking. I know that everybody says that the less unsprung weight the better, but I had no idea that it had this much of an impact. I hate the look of the stockers and love my 18s.... but now I don't even want take the 15s back off!!!"