The tires on the 18's are thicker and thus the total diameter between the two wheels is the same. However, the 18's weigh less. Between 4 and 8 lbs less per corner (yet to be verified). Thats a total unsprung weight savings of 16 to 31lbs... thats about an 80-100 lb difference in sprung weight.... basically like having your skinny girlfriend (really skinny) riding in the car with you. Bottom line, the performance difference is certainly there between the two wheels. Although, a very slight difference... in the end, if your not sure, save 1200 bucks and get the 18's....