The overall tire diameter for a 245/50x16 tire is 25.6. This falls between the 25.3 for a 235/50x16 and 25.7 for a 225/55x16, which are the two "approved" tire sizes for the '91 850. Given that the Euro Csi takes a 245/40x18 up front, I concluded that a 245/50x16 should fit alright. Aside from the +1.6% difference in revolutions per mile, putting on a 245/50x16 seems the way to go, as there is a much greater tire selection available for this size than for 235/50x16. Any feedback will be appreciated.