take a 285/75/16 tire for instance, the 285 is the tread width in millimeters so it would be 285mm, the 75 is the side wall height which is a percentage of the tire width it would be 75% of 285mm and that would be the height of the sidewall, then 16 is the wheel size in inches so the wheel size is 16 inches. does that sense to you? hope this helped. if you were to convert 285/75/16 to inches the tire would roughly be a 32.5x11.25-16 or a 255/85/16 would roughly be a 33x10-16. Hehe been in the tire business to long........