It depends on the tire also, I just bought some OZ racing rims 8.5x18 12x18, I went with Nitto NT-01 and 245/40/18 and 315/30/18 is the closest I could come to a match. Note that the Toyo makes an R888 that is 335/30/18 which is a closer rolling diameter to the 305/30/19 than the 315/30 but Nitto does not make that size. If I was running triple 8's I would go with the 335. on the otherhand if you have a bit of a push in your car then go with the small rear rubber.

 

Did the OZs arrive? I am waiting anxiously for some photos and a report.

 

315/30 x 18 R6 Hoosiers are slightly shorter, but wider than the OEM 19" 305's. If your car is a 997 C2S, the Hoosiers will look smallish. With the GT3, they look perfect at the proper ride height.