Get Tire Size Chart In Inches US . To calculate a metric tire size into inches you multiply the first number by the second number devided by 100 which gives you the sidewall height in millimeters, divide it by 25.4 to convert it to inches. It's a holdover from an old french system that designated tire sizes using the letters a, b and c. Wheels And Tires Sizes Rim Ringz from www.rimringz.com You already have the tire width, the aspect ratio, the rim diameter, that is, you have the full tire size. From overall diameter to revolutions per mile, you’ll also be able to compare tire sizes and learn the proper height, width, and fit when converting tire sizes. Then take the inches and multiply by two and add the rim size. From overall diameter to revolutions per mile, you’ll also be able to compare tire sizes and learn the proper height, width, and fit when converting tire sizes. Note th...