In part two of the tire size series, we explore the second portion of the sidewall description, still using the tire size of a typical passenger car as a reference, P225/50R16 91S. With this detailed description of your tire sidewall information, you'll be able to pick out replacement tires and rims when the time comes for a change. Educating yourself on the important facts about your tires and wheels will make taking care of your vehicle easier. First letter after  Sidewall Aspect Ratio = Internal Construction P225/50R16 91S •    The internal construction is the way the tire body is shaped. •    R= Radial Construction. Following two numbers = Tire and Wheel Diameter P225/50R16 91S •    This number indicates the diameter of the wheel designed to be matched with the tire. •    16 = 16 inch rim diameter. (It's possible to have a rim size that's one digit). Last two numbers & letter = Service Description P225/50R16 91S •    The tire's Load Index (the numbers) and Speed Rating (letter) are described in this last set of indicators. •    91 = a load capacity of 1,356 pounds. •    S = a maximum speed of 112 mph. With this information, you will understand how much your tires can handle and the limits that you should heed when driving. For example, knowing the speed rating will prevent you from overdoing your levels of speed, which could be disastrous for your tires, as well as your life. Likewise, knowing the load index will help you avoid piling your truck bed too high.