235-50-18 is the STOCK tire size on the SS's. My girls SS is actually running a 245-45-18 on the stock rims.
IMO, I would run those 235's till they're bare and then switch to a 245-45. The tires are ALOT cheaper in that size.
They are the same rims that came on the Impala SS's and Monte Carlo SS's, just for some FYI.