"275/35/R20" "315/25/ZR21" these are some of the numbers you see marked on tire side walls. What do they mean and why do they matter? In this article we will explain what they are and how to decipher them.
Step 1: Understand the Basic Markings
The first thing to note is that all tires have a standardized set of markings that provide information about their size, load capacity, and speed rating. These markings are typically located on the sidewall of the tire and include information such as tire size, load index, speed rating, and DOT (Department of Transportation) code.
Step 2: Decipher the Tire Size
The tire size is one of the most critical pieces of information on the tire, as it determines the tire's diameter, width, and aspect ratio. The tire size is usually expressed in a series of numbers and letters. For example, on this front tire for a GTR. the tire size information can be found on the upper left corner "285/35/ZR20" Here, "285" is the nominal tire width in millimeters, "35" is the aspect ratio in percentage (Tire side wall height is calculated as 285mm*35%=99.75mm), and "20" indicates that the tire is meant for a 20-inch wheel. The "ZR" rating on this tire is special however indicating that this tire is capable of handling maximum speed over 149 MPH or 240 km/h.
Step 3: Understand Load Index and Speed Rating
The load index and speed rating determine the maximum weight that the tire can carry and the highest speed at which it can travel safely. Load index is a numerical code that ranges from 71 to 110, where each number corresponds to a specific weight limit in pounds. The speed rating is expressed as a letter from A to Z, with each letter indicating a maximum speed in miles per hour. For example, a tire with a load index of 90 can carry a maximum weight of 1,323 pounds, while a tire with a speed rating of V can travel up to 149 miles per hour. In the above picture, we find the the number is 104Y which means this tire can load up to 1,984 pounds / 900 KG and travel at a maximum speed of 186 mph or 300 km/h.
Step 4: Check the DOT Code
The DOT code is a set of alphanumeric characters that provide information about the tire's manufacturing date, plant code, and tire size. The DOT code is usually located on the sidewall of the tire and is expressed as a series of numbers and letters, For example DOT U2LL LMLR 1521. Here, the last four digits (1521) indicate the tire's manufacturing date, where the first two digits (15) indicate the week, and the last two digits (21) indicate the year.
Step 5: Look for Additional Markings
In addition to the basic markings, some tires may have additional markings that provide information about their specific features and functions. For example, winter tires may have a snowflake symbol or the letters "M+S" (mud and snow) to indicate that they are suitable for use in snowy and icy conditions. Meanwhile, performance tires may have a "Z" or "Y" speed rating to indicate that they are designed for high-speed driving.
In conclusion, understanding the markings on tires is essential for ensuring that you select the right tire for your vehicle and driving conditions. By following these steps, you can decipher the basic markings on tires and make an informed decision when it comes to purchasing and maintaining your tires.