![]() Also, what is missing from this method is anything related to the tire width. Since the load capacity of a tire is a function of the size of the tire, the load range number gave a general idea of how large the tire was, but it was far from perfect. The two-digit number is the aspect ratio, with higher numbers meaning a taller tire, and the number after the dash was the rim diameter in inches. The letter denoted the tire load range, with A being the lowest and getting progressively higher as you move up the alphabet. ![]() To understand this, let’s look at some history.īefore the advent of radial tires, in the days of bias ply tires (which we’ll get to in a sec), size was given by an alpha-numeric system that used a letter followed by a two-digit number, then a dash followed by a two-digit number - something like: F70-15. Even in countries that have used the metric system since it was invented over 200 years, rim diameter is denoted in inches. Width is metric but rim diameter is imperial and it’s true throughout the world. It’s interesting to note that tire size is one of the only instances in the worldwide auto industry where you will see a mix of units. It’s done by multiplying the width by the aspect ratio, times two since there are two sidewalls comprising the diameter, then adding the rim diameter converted to millimeters. These numbers are also very useful if you want to know the overall diameter of your tires. This means the tires are 265 mm wide, have a section height of 265 x 35% = 92.75 mm, and sit on a 20-inch diameter rim. In the case of my Mustang, the tires on this car show 265/35R20. Light truck tires may have “LT” instead of the “P”. You may also see a “P” in front of the tire size which means it is a Passenger tire. The last two digits denote the diameter of the wheel/rim in inches. Take the three-digit tire width, multiply it by this percentage and you get the distance from the rim to the tire tread. The two digits following the slash represent the aspect ratio, which is a percentage multiplier that determines the section height. that authority is the Tire and Rim Association (or T&RA) while in Europe it is European Tyre and Rim Technical Organisation (or ETRTO). Every tire size has a standard rim width defined by the relevant authority. Shown in green, the first three digits are the section width of the tire in millimeters as measured with the tire mounted on a standard width rim. ![]() Shown in red are three digits followed by a slash, followed by two digits, followed by two letters ending in “R” or just the letter “R” by itself, then followed by two more digits. For the rest of you, tire size follows the following format: You probably already know how tire size works, so if you want to skip ahead, you’re welcome.
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