How to Read Tire Size Codes: A Complete Guide
Understanding Tire Size Codes
When you look at the sidewall of your tire, you will notice a series of numbers and letters that might seem confusing at first glance. These markings, such as P225/65R17 91H, contain crucial information about your tire’s specifications. Understanding these codes will help you make better decisions when purchasing new tires and ensure optimal vehicle performance.
Table of Contents
Breaking Down the Tire Size Code
Let us examine a typical tire size code: P225/65R17 91H. Each component of this code provides specific information about the tire’s characteristics.
Tire Type Prefix (P)
The letter at the beginning indicates the tire type. The most common prefixes include:
- P (Passenger): Designed for passenger vehicles including cars, SUVs, and light trucks
- LT (Light Truck): Built for vehicles that carry heavy loads or tow trailers
- ST (Special Trailer): Specifically designed for trailer applications
- C (Commercial): Used on delivery vans and commercial vehicles
If there is no letter prefix, the tire follows European metric sizing standards, which are slightly different from American standards but still compatible with most vehicles.
Section Width (225)
The three-digit number following the prefix represents the tire’s section width in millimeters. This measurement indicates the distance from one sidewall to the other when the tire is mounted and inflated. In our example, 225 means the tire is 225 millimeters wide, approximately 8.9 inches.
Wider tires generally provide better grip and handling but may increase fuel consumption and road noise. Narrower tires often offer better fuel efficiency and perform well in snow conditions.
Aspect Ratio (65)
The two-digit number after the slash is the aspect ratio, expressed as a percentage. It indicates the height of the tire’s sidewall relative to its width. An aspect ratio of 65 means the sidewall height is 65% of the tire’s width.
Construction Type (R)
The letter following the aspect ratio indicates the tire’s internal construction:
- R (Radial): The most common type today, with cord plies arranged radially from the center. Radial tires offer better fuel economy, longer tread life, and improved ride comfort.
- D or B (Diagonal/Bias): Older construction type with cords running diagonally. Still used in some specialty applications.
- F (Run-Flat): Some manufacturers use this to indicate run-flat capability.
Rim Diameter (17)
The number after the construction type indicates the wheel diameter in inches. This measurement must match your wheel size exactly. A tire marked 17 requires a 17-inch wheel. Installing tires on incorrect wheel sizes is dangerous and can lead to tire failure.
Load Index and Speed Rating
Load Index (91)
The load index is a numerical code that indicates the maximum weight each tire can safely support when properly inflated. In our example, 91 corresponds to 1,356 pounds (615 kg) per tire. Common load index values include:
- 85 = 1,135 lbs (515 kg)
- 91 = 1,356 lbs (615 kg)
- 95 = 1,521 lbs (690 kg)
- 100 = 1,764 lbs (800 kg)
Speed Rating (H)
The speed rating indicates the maximum speed at which a tire can safely operate under optimal conditions. Each letter corresponds to a specific speed:
- S: 112 mph (180 km/h)
- T: 118 mph (190 km/h)
- H: 130 mph (210 km/h)
- V: 149 mph (240 km/h)
- W: 168 mph (270 km/h)
- Y: 186 mph (300 km/h)
Additional Sidewall Markings
DOT Code
The DOT (Department of Transportation) code indicates the tire meets safety standards. The last four digits reveal the manufacturing date. For example, 2423 means the tire was produced in the 24th week of 2023.
Treadwear, Traction, and Temperature (UTQG)
The Uniform Tire Quality Grading system provides standardized ratings:
- Treadwear: A comparative rating where higher numbers indicate longer tread life
- Traction: Grades AA, A, B, or C indicating wet braking ability
- Temperature: Grades A, B, or C showing heat resistance at speed
Choosing the Right Tire Size
When selecting replacement tires, always consult your vehicle’s owner manual or the tire information placard located on the driver’s door jamb. This placard lists the recommended tire sizes and inflation pressures for your specific vehicle.
While some drivers opt for plus-sizing (larger wheels with lower-profile tires), this should be done carefully to maintain the overall tire diameter and ensure proper speedometer calibration, adequate load capacity, and correct clearance.
Conclusion
Understanding tire size codes empowers you to make informed decisions about your vehicle’s tires. These markings provide essential information about dimensions, performance capabilities, and safety specifications. Always match your replacement tires to your vehicle manufacturer’s recommendations, and when in doubt, consult a tire professional for guidance.