The metric system has become the convention of choice in advertised tire size. An example of a metric tire size is "P235/75R16". The convention is not particularly user friendly to the average driver and in some instances can be confusing to those unfamiliar with metric tire designations. The metric system was designed in part so that units could be easily converted. However, there's nothing "easy" about unraveling metric tire sizes because not all the figures are dimensions. The 3 digits following the first letter designate the tires width in millimeters. The next 2 digits, which trail the "/", define the aspect ratio; the ratio of the width to the height, measured from top of the wheel to top of the tire. The 2 digits following the next letter is the rim size in inches (yes, in inches). Confused yet? Don't be - use the converter below to quickly calculate nominal tire size in inches.