Tire Code Breakdown

Each tire has an alphanumeric code that provides valuable information about its size, construction, and purpose. Let’s dissect a typical tire code to understand what it means.

1. Intended Use (P, LT, ST, T)

The first letter or letters in the code denote the tire’s intended use:

• P (Passenger): These tires are designed for standard vehicles, such as cars, SUVs, crossovers, minivans, and smaller trucks.
• LT (Light Truck): LT tires are meant for vehicles that need to carry heavy loads or tow trailers. These are often used in larger trucks.
• ST (Special Trailer): These tires are exclusively for trailers, such as boat, utility, or travel trailers.
• T (Temporary): T-rated tires are temporary spares, intended only for short-term use.

2. Nominal Section Width (Three-Digit Number)

The three-digit number following the vehicle class letter indicates the nominal section width of the tire, measured in millimeters. This is the distance from sidewall to sidewall. For example, a tire marked “215” has a nominal section width of 215 mm.

3. Aspect Ratio (Two- or Three-Digit Number)

The aspect ratio follows the section width and represents the tire’s height compared to its width. It’s expressed as a percentage. A tire with a code like 215/65 means that the tire’s height is 65% of its width (in this case, 65% of 215 mm, or 139.75 mm).

4. Internal Construction (R, D, B)

The next part of the code is a single letter that denotes the tire’s internal construction:

• R (Radial): Most modern cars use radial tires, where the internal cords run perpendicular to the direction of travel.
• D (Diagonal): Diagonal, or bias-ply, tires have cords that run at an angle and crisscross over one another. These tires are less common today.
• B (Bias-belted): Similar to diagonal tires, but with added layers of stabilizer belts for extra support.

5. Wheel Diameter (One- or Two-Digit Number)

After the internal construction letter, the next number represents the wheel diameter in inches. This number indicates the size of the wheel or rim that the tire fits. For example, “R16” means the tire fits a 16-inch wheel.

6. Load Index (Two- or Three-Digit Number)

The load index represents the maximum weight the tire can support. A load index of 95, for instance, means the tire can carry up to 1,520 lbs (690 kg) when properly inflated.

7. Speed Rating (One- or Two-Digit/Letter Code)

The final part of the tire code is the speed rating, indicating the maximum speed the tire can handle under its load index. For example, a tire with a speed rating of H is rated for speeds up to 130 mph (210 km/h).

Understanding the DOT Code

In addition to the alphanumeric tire code, tires in the United States must display a DOT code. This identifies the tire’s manufacturer, factory, and the week and year of production. For instance, a code that reads “1321” means the tire was made in the 13th week of 2021. The age of a tire matters, as rubber degrades over time. The National Highway Traffic Safety Administration (NHTSA) recommends replacing tires every six years, even if they still appear in good condition.

How to Calculate Tire Size

Calculating tire size involves two main components: the section height and the overall tire diameter.

• Section Height = Tire Width × Aspect Ratio (as a percentage)
• Tire Diameter = Wheel Diameter + 2 × Section Height

For example, for a tire with code 215/65 R16:

• Section height = 215 mm × 65% = 139.75 mm
• Tire diameter = 16 inches + 2 × 139.75 mm = 25.48 inches

This information is crucial when determining the right tire size for your vehicle.

Changing Tire or Wheel Sizes: Key Considerations

Many drivers choose to change their tire or wheel sizes, often for aesthetic purposes. However, it’s essential to consider several factors before doing so:

• Suspension and Handling: Changing the tire size can affect the vehicle’s suspension and brake components, particularly in vehicles with anti-lock braking systems (ABS).
• Speedometer and Odometer Accuracy: Larger tires increase the wheel’s total circumference, causing the speedometer to show a lower speed than the car is actually traveling. Conversely, smaller tires will make the speedometer show a faster speed than actual. This change also affects the accuracy of the odometer.

When switching tire or wheel sizes, it’s recommended to choose sizes that differ by no more than 3% from the manufacturer’s specifications.

Final Thoughts

Tires are more than just rubber on wheels; they’re a critical component of vehicle safety, performance, and efficiency. Understanding tire codes can help you make informed decisions about your tires, ensuring that your vehicle remains safe, comfortable, and optimized for performance. Whether you’re replacing tires due to wear and tear or upgrading your rims, knowing how to read tire codes is the first step to selecting the right tires for your vehicle.

By carefully considering factors like load index, speed rating, and tire size, you can maintain your vehicle’s performance while avoiding costly mistakes. Remember, proper tire maintenance not only enhances your driving experience but also prolongs the life of your vehicle.