Since you are posting this in the racing forum, I can only assume you are talking about racing your SC?
If so, and you plan to race with PCA, keep an eye on the rule changes for 2011. There has been a trend towards the 15" setup with Hoosier R6 tires that results in a lower drive ratio as Bill points out. There is a proposal to change the PCA rules penalizing that wheel/tire combo.
Wheels and tires are funny. As I understand it, 16" rims were a popular "upgrade" when 911SCs were new. When I started racing in 2006, all the fast cars were going with 17" rims, the theory being a lower side wall height resulted in less tire flex. Those that could afford high-dollar Fiske 17" forged wheels were the envy of everyone at the track.
But ow 15" is the trick setup. Unless the rules change, and then it won't be anymore.
