Understanding the information on your tire sidewall is crucial for safety and informed vehicle maintenance. Among the many markings, knowing how to read your tire size is fundamental. This guide will break down the components of tire size and delve into other important tire ratings, ensuring you have a comprehensive understanding of your tires.
Decoding the Tire Size
The tire size is typically displayed as a combination of letters and numbers, such as P225/50R17 98H. Each part of this code provides vital information about the tire’s dimensions and intended use. Let’s dissect each component:
1. Tire Type (Optional Prefix)
The first letter, if present, indicates the tire type:
- P: P-metric tire, commonly used for passenger vehicles in the US.
- LT: Light Truck tire, designed for pickup trucks, SUVs, and vans.
- ST: Special Trailer tire, intended for trailers.
- If there is no letter, it usually indicates a Euro-metric tire. P-metric and Euro-metric tires are generally interchangeable, but it’s always best to consult your vehicle’s owner’s manual.
2. Tire Width
The three-digit number following the tire type represents the tire width in millimeters. In our example, 225 indicates the tire is 225 millimeters wide at its widest point from sidewall to sidewall.
3. Aspect Ratio
The two-digit number after the slash is the aspect ratio. 50 in our example means the tire’s section height is 50% of its width. To calculate the section height, you would take 50% of 225mm. A lower aspect ratio means a shorter sidewall, often associated with better handling, while a higher aspect ratio provides a more comfortable ride.
4. Construction Type
The letter following the aspect ratio indicates the tire’s internal construction:
- R: Radial construction. Almost all modern tires are radial, meaning the cords that reinforce the tire are arranged radially across the tire from bead to bead. Radial tires offer better fuel economy, ride comfort, and tread life compared to older bias-ply tires.
- B: Bias-ply construction. Older type of construction where the ply cords run diagonally across the tire.
- D: Diagonal or bias-belted construction. Similar to bias-ply but with belts added for stabilization.
5. Rim Diameter
The two-digit number after the construction type represents the wheel rim diameter in inches. 17 indicates that this tire is designed to fit a wheel with a 17-inch diameter. It is crucial to match the tire’s rim diameter to your vehicle’s wheel size.
6. Load Index
The number following the rim diameter is the load index. 98 in our example is a numerical code that corresponds to the maximum weight each tire can carry when properly inflated. You need to consult a load index chart to find the actual weight capacity. For instance, a load index of 98 corresponds to 1653 lbs (750 kg). Always ensure the tires you choose meet or exceed your vehicle’s required load index.
7. Speed Rating
The final letter in the tire size code is the speed rating. H in our example indicates the maximum speed at which the tire is designed to be safely driven for extended periods. Speed ratings range from L (lowest) to Y (highest), with each letter corresponding to a specific speed in miles per hour (mph) or kilometers per hour (km/h). An ‘H’ speed rating corresponds to a maximum speed of 130 mph (210 km/h). It’s crucial to select tires with a speed rating that meets or exceeds your vehicle’s top speed capability and typical driving conditions.
Understanding UTQG Ratings: Beyond Tire Size
While tire size provides dimensional information, the Uniform Tire Quality Grading (UTQG) system offers insights into a tire’s performance characteristics. Required for passenger car tires (excluding winter tires and light truck tires), UTQG ratings help compare tires based on:
1. Treadwear
The Treadwear grade is a comparative rating based on the tire’s wear rate when tested under controlled conditions on a government test course over 6,000 miles. A tire graded 150 is projected to last 1.5 times longer than a tire graded 100 under the same test conditions. However, real-world tread life is influenced by driving habits, road conditions, and tire maintenance.
2. Traction
Traction grades, ranging from AA (highest) to C (lowest), indicate a tire’s wet braking performance in straight-line conditions on asphalt and concrete surfaces. It’s important to note that the traction grade only measures straight-line wet braking and does not assess cornering traction. A tire with an AA rating will provide superior wet braking traction compared to a tire with a C rating.
WARNING: THE TRACTION GRADE ASSIGNED IS BASED ON A WET BRAKING (STRAIGHT AHEAD) TRACTION TEST AND DOES NOT INCLUDE CORNERING (TURNING) TRACTION.
3. Temperature
Temperature grades, A, B, and C, represent a tire’s resistance to heat generation and its ability to dissipate heat when tested under controlled laboratory conditions. Heat buildup can degrade tire materials and potentially lead to tire failure. Grade C is the minimum standard all passenger car tires must meet. Grades A and B signify higher levels of heat resistance.
WARNING: THE TEMPERATURE GRADE IS ESTABLISHED FOR A TIRE THAT IS PROPERLY INFLATED AND NOT OVERLOADED. EXCESSIVE SPEED, UNDER INFLATION, OR EXCESSIVE LOADING, EITHER SEPARATELY OR IN COMBINATION, CAN CAUSE HEAT BUILDUP AND POSSIBLE TIRE FAILURE.
DOT Compliance
In addition to UTQG ratings, passenger car tires must also meet other federal safety requirements. The Department of Transportation (DOT) symbol on the sidewall indicates that the tire meets or exceeds these safety standards.
Conclusion
Understanding How To Read Tire Size and interpret UTQG ratings empowers you to make informed decisions when selecting tires for your vehicle. While tire size ensures proper fitment, UTQG ratings provide a comparative measure of tire performance. Always consult your vehicle owner’s manual for recommended tire size and specifications and seek advice from tire professionals for personalized recommendations based on your driving needs and conditions.
