How to Read the Sidewall of a Tire

The markings stamped into a tire's sidewall may seem like indecipherable code. But these inscriptions are anything but arbitrary. It’s a wealth of information, telling you everything about your tires from the production date and designed maximum speed, to their expected lifespan.

Let’s demystify the sidewall of a tire, breaking down each number and symbol and learning how to interpret them. This isn’t just technical jargon. It's practical information that impacts vehicle performance and safety.

So, whether you're a new driver or a seasoned enthusiast, let's dive in and turn that jumble of letters and numbers into practical and actionable information.

Tire Size and Type

One of the most prominent tire markings reveals its size and type. This sequence often begins with "P" or "LT", followed by a series of numbers and letters. For example, you might see: "P250/50R17". Here's a breakdown of the components:

• P: This stands for "passenger", meaning the tire is designed for passenger vehicles like cars, crossovers and minivans. You may also see "LT", which stands for "light truck", indicating the tire is suitable for pickups and larger SUVs that carry heavier loads.
• 250: This represents the tire's width in millimeters from one sidewall to the other. Wider tires provide more cornering grip, while narrower tires offer better fuel economy.
• 50: This is the aspect ratio, a percentage comparing the tire's sidewall height to its width. In this example, the tire's sidewall height of 125 millimeters is 50% of its 250-millimeter width. Lower numbers indicate shorter sidewalls, which improve responsiveness. Higher numbers equate to taller sidewalls, which offer more ride comfort.
• R: This stands for "radial," which refers to the tire's construction type with the cordplies arranged at 90 degrees to the direction of travel, or radially, from the center of the tire. For passenger vehicles, radial tires are the standard construction.
• 17: This indicates the diameter of the wheel, in inches, that the tire is designed to fit. In this case, the tire would fit a 17-inch wheel.

Understanding the size and type of the tire is crucial for selecting the right tire for your vehicle and ensuring its compatibility with your car’s engineering, purpose and weight. Getting it wrong can impair both vehicle performance and safety.

(source: istockphoto.com)

Performance Specifications

Near the tire's size and type markings, you should also find its load index and speed rating. This appears as a number and letter combo, such as "91V." The numerical value is the load index, which ranges from 70 to 126 and indicates the maximum weight the tire can support. A load index of 91, for example, corresponds to 1,356 pounds for each tire (or 5,424 pounds for the entire vehicle).

Similarly, the letter in that sequence denotes the tire's speed rating. Ranging from L to Z, it tells you the maximum speed at which the tire is designed to operate. Most passenger cars have tires with a speed rating of at least S (112 mph). Y-rated tires are the highest tire speed rating, good for up to 186 mph.

Near the tire's size, you'll see a marking like "Max. Press. 35 PSI", indicating its maximum air pressure. Note that this is usually higher than the pressure the automaker recommends, which is specific to each vehicle and can be found in the owner's manual or on a sticker located in the driver's door jamb.

Like the tires used in Formula 1 racing, some maximum high-performance tires are directional tires, which means they are designed to rotate in a specific direction for optimal performance. If so, you'll see an arrow on the sidewall indicating the direction it should rotate when moving forward. Installing such tires in the correct orientation is crucial for achieving their designed performance and safety levels.

Treadwear, Traction and Temperature

Since the late 1970s, the National Highway Traffic Safety Administration (NHTSA) has required tiremakers to display treadwear, traction and temperature ratings on all tires sold in the U.S. These ratings, known as the Uniform Tire Quality Grading (UTQG) system, give shoppers a common standard for comparing different tire models and brands across three areas:

• Treadwear: This number indicates the tire’s expected lifespan compared to a baseline. If the tire matches the lifespan of the reference tire, it receives a UTQG rating of 100. Tires that last twice as long have a 200 rating and so on. The higher the number, the longer the tire should last. For example, a tire with a treadwear rating of 400 should last twice as long as one with a 200 rating.
• Traction: Represented by grades such as AA, A, B or C, this rating reflects the tire's performance on wet pavement. The highest grade, like AA, means the tire should handle and stop better in rainy conditions.
• Temperature: This grade -- A, B or C -- indicates the tire’s resistance to heat build-up. Higher grades mean the tire can better withstand and dissipate heat, which is important for performance driving or for vehicles driven in hot climates.

Tires with an "M+S" or "M/S" designation are all-season tires that are also capable of driving in slightly muddy or snowy conditions. These shouldn't be confused with dedicated off-road or snow tires, however.

Tires designed for off-roading, also called all-terrain tires, are usually denoted with "A/T" or "T/A" markings on the sidewall, while tires with excellent snow and ice performance display a symbol of a mountain with a snowflake inside. Known as the three-peak mountain snowflake (3PMSF), this symbol indicates the tire meets specific performance requirements for driving in severe winter weather.

Date of Manufacture

A tire's date of manufacture is another crucial piece of information stamped into its sidewall. It's found in the last four digits of a code that begins with the letters "DOT" (for "Department of Transportation"). The first two digits represent the week of the year the tire was made, while the last two represent the year. If you saw "0120" for example, it would mean the tire was manufactured in the first week of 2020.

Knowing the tire's date of manufacture allows you to calculate its age, which is important because tires deteriorate over time, even if they're not used. Temperature, sunlight and humidity degrade the rubber compounds in the tire over time, compromising performance and safety. As a result, experts recommend replacing tires that are more than six years old, regardless of how much tread remains.

(source: automoblog.com)

Final Thoughts

As the only part of your car connected to the road, tires play a key role in every aspect of its performance as well as its safety. That's why, from size, to the tire’s ratings, to the date of manufacture, it's vital to know and understand the info embedded on the sidewall.

Armed with this knowledge, you'll not only be able to select the right tires for your vehicle, but understand exactly when it's time to replace them.