Tires are essential in keeping your vehicle maneuverable on the road. They’re made of vulcanized rubber—which is rubber that has been hardened and cooked—so that they can grip the ground and provide suitable traction on various road surfaces. They typically differ in terms of vehicle, driving, and weather applications. A tire that’s meant to fit a daily-driver passenger car is different from a tire that would fit a light truck or an exotic sports car.
In case you didn’t know, vehicle tires are categorized according to tire speed rating. This rating follows an alphabetical assignment ranging from B to Z. Each letter has a maximum speed designation and you’ll usually see these letters on the walls of your tires, along with a series of numbers.
What Does Tire Speed Rating Mean on Tires?
The durability of a tire varies depending on the tire’s speed rating, which refers to the maximum speed a tire can safely carry the vehicle’s weight and payload in ideal conditions. Tire speed ratings come in the form of a letter that follows the load index value of the tire.
The most common speed rating letters range from L to Y.
Tire Speed Rating Chart
To get a clearer understanding of tire speed rating, here’s a chart to help you.
Notice that Z comes right before W and Y. That’s because Z-speed tires were meant to represent speed ratings higher than 149 mph. W and Y-speed tires were innovated later as cars got faster with the help of technology.
A more recent development is a tire with a (Y) rating, which is given to a tire that can withstand high speeds past the 186-mph mark. Check the chart below:
|Speed Rating||Maximum Speed|
What is Tire Load Index?
The numerical value found preceding the speed rating is referred to as the tire load index. It indicates the tire’s capacity to safely carry a load under maximum air pressure. The numbers correspond to actual weight measurements typically ranging from 739 lbs to 3,748 lbs for load index values, starting at 70 and going all the way up to 126.
Combined, the speed rating and load index are known as the tire servicing description.
Tire Load Index Chart
Keep in mind that the tire load index is crucial when shopping for a replacement tire. The tire must be able to support your vehicle’s gross vehicle weight rating (GVWR). If you can’t find the tire servicing description on the sidewall, you may refer to the owner’s manual or visit a specialist for proper tire evaluation.
For a quick guide or reference, here’s a table of load index values and the corresponding load measurements in pounds:
|Load-Carrying Capacity Index Ratings|
|Load Index||Load (lbs)|
How to Read Tire Numbers
If you look closely at the tire wall, you’ll see something similar to P215/65R17 98T. This label follows a standard arrangement of numbers and letters separated by a forward-slash (X000/00X00 00X). It may look intimidating at first, but it’s actually quite simple to decode. Breaking it down should help you understand more about your tire and help you in acquiring a replacement.
The ‘P215’ on the first part consists of two separate pieces of information. The ‘P’ stands for “passenger vehicle,” which means the tire can fit sedans, hatchbacks, minivans, and light-duty pickup trucks. The letter can vary, depending on whether the tire is for light trucks, a temporary spare, or for special trailer uses—in which case it could be LT, T, and ST, respectively. Remember that knowing the right trailer tire speed rating is essential, especially if you tow on a regular basis.
Meanwhile, the ‘215’ refers to the section width (in millimeters) of the tire from the widest points of the inner and outer sidewalls.
The values succeeding the forward-slash consists of the aspect ratio, tire construction, tire diameter, tire load index (98), and tire speed rating (T). Using the same example above, 65 is the aspect ratio of the tire while 17 is the tire diameter (in inches).
Aspect ratio, also known as the tire profile measurement, indicates the thickness of the sidewall from the rim to the outside tread of the tire. The 65 value means the tire has 65% of the section width.
Tire or wheel diameter is the overall measurement of the tire from two opposing outer tread points.
The letter in between the aspect ratio and tire diameter refers to the internal construction of your tire. There are three different tire constructions, namely radial (R), bias-ply/diagonal (D), or belted (B). Radial is the most common tire construction of the three and features cord plies oriented at 90 degrees to the direction of travel. Bias-ply tires have cords that extend diagonally at 30 or 40-degree angles. Lastly, belted tires use circumferential belts at different angles placed on top of two main plies.
What P215/65R17 98T means:
|Tire Class (P)||This first letter indicates the class of the tire.|
P = Passenger vehicle
LT = Light Truck
T = Temporary Spare
ST = Special Trailer
|Section Width in mm (215)||This is the measurement of the tire’s width from sidewall to sidewall in millimeters.|
|Aspect Ratio (65)||This will tell you the height of the sidewall. It is measured as a percentage of the section width. A value of 65 means that the height of the sidewall is 65% of the section width. In this case, that would be 139.75 mm|
|Internal Construction (R)||This tells you what type of construction was used in fabricating the tire.|
R = radial
D = bias-ply/diagonal
B = belted
|Wheel Diameter in inches (17)||This is the overall measurement of the tire from two opposing outer tread points in inches.|
|Load Index (98)||This indicates the maximum loading capacity of the tire. Make sure to only install tires that meet or exceed the load index specified for your vehicle.|
|Speed Rating (T)||This is indicative of the maximum speed a properly installed and inflated tire can be driven on.|
How Do Tire Manufacturers Come Up with Speed Ratings?
A tire’s speed rating is determined by putting the tire through a series of tests using a specifically designed machine. The goal is to meet or surpass the Economic Commission for Europe (ECE) standards. Some tires even undergo more rigorous tests to comply with the Society of Automotive Engineers (SAE).
In an ECE test, the tire is inflated to the factory-recommended PSI on a wheel attached to a testing machine. Tests are performed inside a room that is heated to a temperature of 77 degrees Fahrenheit.
Tests include a load and speed test, visual evaluation, and an optional set-up speed (SUS) test. In a load test, the tire is inflated to factory PSI and pressed against a metal drum to imitate realistic load conditions. After the load test, the tire is run from 25 mph to a designated target speed for 10 minutes. The tire is then removed from the mount and visually inspected for signs of tread wear.
Some tires may also undergo the more intense SAE test where it is run inside a 100-degree testing facility.