I am considering changing the wheels on my 997S from 19" to 18". I have always liked the racy look and performance of thicker sidewalls as opposed to low profile and harsh ride. Some people say the car's suspension was designed for 19" wheels and that 18" wheels would compromise handling/cornering. I did some homework and found that 235/40/18 has about the same tire diameter and turns per mile (25.4 & 819) as the stock 235/35/19 (25.5 & 817) and for the rears the stock is 295/30/19 (26 & 799)... I could use 295/35/18 (26.1 & 797) or 315/30/18 (25.5 & 817). I am not a tire expert but I am assuming that if I use an 18" tire that has an equivelant diameter and turns per mile as the stock 19" tires I will have equal performance. Is this true? Sorry to pick your brains and hope someone with tire/wheel/suspension experience can educate me.