Stock rear tires for my 968 are 255/40/17's, but such low aspect ratio tires ride HARD over the bumps. Just the jump to 45 aspect ratio tires (like the fronts) would make a difference , I feel. Years ago the 255/45/17 size weren't available for many tires, so is this the reason for the low aspect tire requirement in the rear only? Or is there a clearance problem? Continental Extreme Contact performance summers are available in that size now. Any input appreciated. Anybody ever try this?