Quick tire sizing lesson. Some simple math will help to figure out the over all sizes of your tires. First of all, the width of any modern tire is measured in millimeters. Use this formula to convert from millimeters to inches: Millimeters divided by 25.4 (25.4 millimeters equals 1 inch). If your tire size reads 255/60/R15, ten your tire is approximatle 10 inches wide (at it's widest point). 255 / 25.4 = 10.03937. The next number, 60, is a percentage of the tires width for the sidewall. In other words, the sidewall is 60% of the tires width. Since we know the tires width is 10 inches, we can take 60% of that to figure the height of the sidewall, approx. 6 inches. We can also figure the overall height of the tire by doubling the sidewall height (top of the tire plus bottom of the tire) and adding the rim's diameter. We know that the sidewall is 6 inches tall, so we multiply that by 2 to get 12 inches, then add 15 inches for the diameter of the rim to get 27 inches. This will work for any modern tire that is described in sizes using numbers. The old letter system is alot more confusing. Please note that the second number in the size description is not a set size, but is a variable. That is why a 215/70/R15 will be close to the same diameter as a 255/60/R15. Of course all these sizes are approximates as there are variables between manufacturers. Often times, a manufacturer can give you a good idea of all the tire sizes if you explain to them what size tires you want on what size rim. Do the research first and save yourself alot of headaches later.