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One skill every car owner should have is the ability to read a tire's sidewall.
A tire can tell you its size, as well as other information, such as its construction, speed rating, and load capacity (to name a few). That's why Palacio Tire and Muffler Shop has included this quick and handy guide on how to read a tire.
Throughout the guide, we'll use the example "P 225/55R18 97T" and bold the letter or number being discussed.
P 225/55R18 97T
Many tires have a letter at the beginning that tells you the tire type.
You may come across the following:
The example tire is a passenger tire.
The three-digit number after the letter is the tire width, from sidewall to sidewall, in millimeters.
In the example, the width of the sidewall is 225 millimeters. Tire width is typically measured and expressed in millimeters for two reasons. The first is because it is easier to use millimeters than inches with tire width. For example, the exampe's tire's width becomes 8.8582" when converted to inches. The second reason is because the metric system, which uses millimeters, is the globally recognized system for tire measurement.
The aspect ratio comes next, separated by a slash. This tells you the height of the tire (from the wheel rim to the outermost point of the tire's tread) in relation to the width of the tire, as a percentage. In the example, the height of the tire is 55% of 225 mm.
The aspect ratio helps determine the tire's overall profile and can impact its handling characteristics, ride comfort, and ability to absorb road imperfections.
The letter that follows the aspect ratio tells you about the construction of the tire. Examples include:
B - Belted Bias: Belted bias-ply tires have plies that crisscross at an angle (typically about 30 to 40 degrees) and a reinforced, diagonal belt.
D - Diagonal Bias: Diagonal bias tires have plies that run diagonally from one bead of the tire to the other and cross at a steeper angle (usually 45 degrees).
F - Runflat: Tires with run-flat construction can support the vehicle's weight temporarily after a loss of air pressure.
ZP - Zero Pressure: This denotes a type of run-flat tire that can operate for extended distances, even after losing all air pressure.
The tire in the example is radial, which is common to see with passenger tires.
P225/55R18 97T
After the type of construction is the wheel diameter.
This number is the diameter of the wheel or rim, in inches, that the tire is designed to fit.
You'll remember from earlier that a tire's width is expressed in millimeters. The practice of using inches for wheel diameter while using millimeters for tire width is the result of historical conventions and standards in the automotive industry
In this example, the tire is intended for a wheel with an 18-inch diameter.
The load index comes next. This is the maximum weight capacity that the tire supports. This number is symbolic. It corresponds with a larger number, but you need to consult a tire manufacturer's load index chart to get it. The load index chart lists the corresponding weight capacities in pounds or kilograms. For example, a load index of 97 typically corresponds to a maximum load capacity of 1,609 pounds (or 730 kilograms). Note that load index charts can vary slightly between tire manufacturers, so for the most accurate number, check with the tire manufacturer.
Next to the load capacity is a letter that tells you the maximum speed a tire can go before there is an impact on its performance.
Here are some of the letters you might come across and what they mean:
In the example, the speed rating is 118 miles.
Speed ratings might seem arbitrary or "all over the place" when looking at miles per hour. If you're wondering why they don't all end in 0s, try converting miles per hour to kilometers per hour. For example, R typically represents a maximum speed of 106 mph or 170 km/h and V corresponds to a maximum speed of 149 mph or 240 km/h.
There are other letters and symbols you might run across on a tire. A tire that has a "M+S" marking on it, for example, means that the tire meets the minimum requirements for use in mud and snow. When there are accidents during winter months, Highway Patrol officers look to see if the vehicles involved have M+S tires installed or not.
Be mindful that a tire with the M+S marking doesn't mean it will perform well in severe conditions. If you will be driving in an area with frequent, severe weather conditions, consider using a dedicated tire designed for those challenges.
Some tires have a three-peak mountain and snowflake (3PMS) marking. A tire with the 3PMS symbol meets the minimum requirements for traction in snow. Note that is is possible to run across different types of tires that meet this minimum standard.
Some tires are directional. This means they must be rotated a specific way. The sidewall of a directional tire may have an arrow and the word "Rotation" or "Direction" to indicate the tire's forward position. If a directional tire is mounted incorrectly, it won't work as intended.
Tires can also be asymmetrical, having an inside and outside to their tread. Many performance tires are asymmetrical. If you have a asymmetrical tire, it should be indicated somewhere on the sidewall. Your tires might be marked "Inside", "Outside", or "This side out".
Congratulations! You know how to read a tire, so there's no better time to test your knowledge than with your own set. Go take a look at your tires and see what they tell you. While you're looking at your tires, don't forget to check that they are properly inflated and examine the tread. You should be regularly checking your tires so you can keep them at their best and catch any issues early.
If you need assistance, the team at Palacio Tire and Muffler Shop is happy to help. Join the tanks of our satisfied customers across New Braunfels, TX, San Marcos, TX, Seguin, TX, and surrounding areas.
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