Tire Code

Car tires are labeled with an alphanumeric code that conveys important information about their specifications. This code is crucial when selecting new tires or wheels as it helps determine tire size and compatibility.
Tire marks
Tire marks / Photo credit by

Considering tire marks, you’ll face with the key elements within the tire code:

  • Intended Use: The code’s opening segment consists of 1 or 2 letters (P, LT, ST, T) indicating the intended vehicle type. There are four categories:
    – “P” designates passenger cars, suitable for various vehicles, including cars, SUVs, crossovers, minivans, and smaller pickups.
    – “LT” stands for light trucks, tailored to handle heavy loads and towing trailers.
    – “ST” refers to special trailer tires used for trailers like boat trailers, utility trailers, and travel trailers.
    – “T” signifies temporary spare tires, not meant for regular use.
  • Nominal Section Width: The following part contains a 3-digit number signifying the tire’s nominal section width in millimeters (mm). This width extends from one sidewall to the other, with “215” indicating a width of 215 mm.7
  • Aspect Ratio: Following a forward slash (/), you’ll find a 2- or 3-digit number that represents the tire’s aspect ratio. This ratio compares the section height (from rim to outer edge) to the width. For instance, “xxx/65” implies the section height is 65% of the tire’s width, e.g., “200/65” results in a section height of 130 mm.
  • Internal Construction: A single letter (B, D, R) in the code signifies the tire’s internal build. This construction, often omitted, plays a crucial role in strength and durability:
    – “R” stands for radial, typical for passenger cars. Cords run circumferentially, perpendicular to the direction of travel.
    – “D” denotes diagonal construction, with cords at a 30-40° angle.
    – “B” indicates bias-belted tires, similar to diagonal but with an added layer of stabilizer belts for extra support.
    – Cross-ply tires feature nylon cords at approximately 55° angles.
  • Diameter of the Wheel: A 1- or 2-digit number in the code specifies the wheel diameter, measured in inches, for which the tire is designed.
  • Load Index: The code also includes a 2- or 3-digit number known as the load index, indicating the tire’s maximum weight capacity at a specific inflation level. For example, a load index of 95 means the tire can carry 1,520 lb (690 kg) at 42 psi.
  • Speed Rating: The code concludes with a 1- or 2-digit/letter combination denoting the tire’s speed rating. This rating defines the tire’s maximum safe speed while carrying a load corresponding to its load index. For instance, a tire with a load index of 95 and speed rating “H” can manage 1,520 lb (690 kg) at 42 psi, traveling at speeds up to 130 mph (210 km/h).

A tires may feature additional markings like the DOT (Department of Transportation) code, mandated in the United States. The DOT code reveals manufacturing details, including the production date. It’s crucial to consider the production date when purchasing new tires, as tires older than 10 years should not be used due to material aging. The NHTSA recommends tire replacement every six years, regardless of mileage.

How to Calculate the Tire Size?

Ever wondered how tire size is determined? It’s a precise process involving specific equations.

Section Height. This is the height of the tire’s cross-section, from the rim to the outer edge. It’s computed using the aspect ratio and tire width from the tire code. The tire width, found before the forward slash (/) in the code, is a 3-digit number in millimeters. The aspect ratio, the 2- or 3-digit number following the forward slash, is crucial. To calculate section height, you simply multiply the aspect ratio by the tire width:

Section Height = Aspect Ratio × Tire Width

Tire Diameter. Once the section height is determined, you can calculate the tire’s overall diameter. This diameter tells you how big the tire is. To calculate it, you need to consider both the wheel diameter and the section height. The wheel diameter is usually a 1- or 2-digit number that appears after the letter indicating the tire’s internal construction. In some cases, the internal construction isn’t noted, and the wheel diameter directly follows the aspect ratio in the tire code. This wheel diameter is measured in inches. To get the tire diameter, you add twice the section height to the wheel diameter:

Tire Diameter = Wheel Diameter + 2 × Section Height

Key Considerations When Altering Tire/Wheel Size

There are various motivations for individuals to modify the size of their tires and wheels, often driven by aesthetic preferences. However, making such changes requires thoughtful consideration due to potential consequences. Failure to do so can lead to significant issues, including heightened wear and tear on a vehicle’s suspension and braking components, especially anti-lock brakes. These consequences have the potential to impact a vehicle’s handling and even cause mechanical problems affecting the transmission.

It’s important to note that there is a wide array of tire sizes available, and different sizes can often be fitted onto the same rim, provided the central portion of the tire matches the correct size. A common alteration involves increasing the size of the rims, but this change necessitates adjustments in tire size as well. Typically, larger rims should be paired with narrower tires to ensure they fit properly within the wheel well of the vehicle. As a general guideline, it’s recommended to choose tires with a diameter that doesn’t deviate more than 3% from the factory-recommended tire size.

Tire replacement may also be prompted by blowouts or wear and tear. In such cases, it’s ideal to replace all tires simultaneously. If this isn’t feasible, it’s advisable to change either the front two or rear two tires together, depending on the location of the tire that requires replacement. When replacing only one or a few tires, it’s strongly recommended to select tires of the same type and brand. Variations in tire characteristics can lead to numerous issues, including compromised control, risk of spinouts while driving, inaccuracies in speedometer readings, or even potential damage to the vehicle’s stability system calibrations.

It’s crucial to understand that altering tire size also has implications for a vehicle’s speedometer and odometer. These instruments are calibrated based on the specifications of stock tires. Consequently, using different-sized rims and tires can result in changes to the vehicle’s behavior. Larger rims and tires increase the total circumference of the wheel, leading to variations in the distance covered per revolution of the tire. Consequently, this leads to discrepancies in speedometer and odometer readings. Specifically, larger tires yield lower speedometer readings than the actual speed, as well as lower odometer readings for the distance traveled. Conversely, smaller tires result in higher speedometer readings and odometer values. Therefore, any alterations in tire and wheel size should be made with a full awareness of these potential impacts on vehicle performance and accuracy.