Let's look at a tire sized at P215/60R16-94T. This is an average tire size you might consider for a '67 Mustang. The first number, 215, is the tread width in millimeters. The next number, 60, is called the aspect ratio. Here, sidewall height is expressed as a percentage of tread width. In this case, it's 60 percent, or slightly more than half the width of the tread. The R16 tells you that it's a radial tire made to fit a wheel 16 inches in diameter. Understanding the tire size code will enable you to purchase exactly the right sized tire for your needs.