Different tire manufactureres have different tread designs/shapes. One 225/40 18 could have squared edges, one could have rounded edges, one could have the "curb protection" edges and so on. All those different variance will lead to different "section widths" which is the widest point of the tire generally..............not the tread width. A quick way to tell if/where it's rubbing is to turn the wheels all the way and check the wheel well for a "clean" spot with a flashlight.