It should be the same wherever you measure your tyre’s width across. R is the radial profile of a tyre, meaning the size of the radius measured on the wall when inflated. When comparing tires with identical aspect ratios, a 285 tire typically measures 0. The height difference becomes more pronounced with different aspect ratios – a 275/65R17 versus a 285/70R17 shows approximately 1.This number refers to the section width of the tire, measured in millimeters from sidewall to sidewall; thus, a 255 tire is 10mm wider than a 245 tire. This is the aspect ratio; the sidewall height of the tire as a percentage of its width.