you can run whatever diameter you want from 13" to 14" as I read the 2006 GCR:
"7. Wheels/Tires
a. Any wheel/tire may be used within the following
limitations:
1. Cars originally equipped with twelve (12) inch wheels
may fit thirteen (13) inch wheels. Cars originally
equipped with metric 365 wheels may fit fourteen (14)
inch wheels, and cars originally equipped with metric
390 wheels may fit fifteen (15) inch wheels. The abovementioned
cars as well as those cars originally equipped
with thirteen (13) inch or fourteen (14) inch wheels
may fit up to a fifteen (15) inch wheel. Cars may not
fit wheel diameters smaller than those listed on their
spec line......"
i personally have some 14x6 wheels new from tire rack
http://www.tirerack.com/servlet/CallJsp?ta...Special=Pricing
and some 13x6 used panasports. the 13" gives better gearing at some tracks. the compromise on the cheaper 14" made it easier to buy the panasports.
