Understanding Tyre Sidewall Markings: A Practical US Guide

Introduction

Every tyre on the road carries a wealth of information right on its sidewall. These alphanumeric markings, often overlooked or misunderstood, are critical to selecting the right tyre for your vehicle and driving needs. In the US, understanding these codes goes beyond basic size—it’s about safety, legal compliance, and optimizing your driving experience. Whether you’re a daily commuter, a fleet manager, or simply replacing a worn-out set, decoding tyre sidewall markings ensures you make informed, confident decisions.

Decoding the Tyre Size Marking

Standard Size Format Explained

The most prominent marking is the tyre size, typically in a format like 205/55 R16 91V. Here’s what each part means:

• 205: Section width in millimeters (width of the tyre from sidewall to sidewall).
• 55: Aspect ratio (height as a percentage of width; here, 55% of 205mm).
• R: Radial construction (virtually all modern passenger tyres).
• 16: Wheel diameter in inches (the rim size the tyre fits).
• 91: Load index (numerical code for max load capacity).
• V: Speed rating (maximum speed capability).

US vs. Euro Markings

US tyres follow the same basic format as European tyres but may also list P-Metric (P for passenger), LT (light truck), or T (temporary spare) designations. For example, P215/65R15 indicates a passenger vehicle tyre, while LT245/75R16 is for light trucks.

Load Index and Speed Rating

Load Index: What It Means

The load index is a numerical code that tells you the maximum weight each tyre can carry when properly inflated. For example, a load index of 91 corresponds to a max load of 1,356 lbs (615 kg) per tyre. Always consult the load index chart (available from tyre manufacturers and on the NHTSA website) to match your vehicle’s requirements.

Speed Rating: Legal and Practical Limits

The speed rating letter (e.g., T, H, V, W) indicates the maximum speed a tyre can handle under specified conditions. For instance:

• T: up to 118 mph
• H: up to 130 mph
• V: up to 149 mph

Choose a tyre with at least the speed rating recommended by your vehicle manufacturer. Using a lower speed-rated tyre can affect handling and may not be legal in some states.

DOT Code and Tyre Age

The DOT Code

The Department of Transportation (DOT) code is crucial for safety and recalls. The code looks like DOT X0 XX XXXX 2522. The last four digits represent the week and year of manufacture—in this example, the 25th week of 2022.

Why Tyre Age Matters

Tyres degrade over time, even if tread remains. Most manufacturers and safety advocates recommend replacing tyres that are over six years old, regardless of tread depth. Always check the DOT code when purchasing new or used tyres to ensure freshness.

Other Important Sidewall Markings

Maximum Pressure and Load

Tyre sidewalls will list the maximum inflation pressure (e.g., “Max Press 44 PSI”) and load (e.g., “Max Load 1,477 LBS”). This is not the recommended running pressure for your vehicle—always use the pressure listed in your owner’s manual or driver-side door placard.

M+S, 3PMSF, and Other Performance Markings

• M+S: Mud and Snow; meets basic all-season traction requirements.
• 3PMSF (Three-Peak Mountain Snowflake): Meets severe snow traction standards (important for winter tyres in northern states).
• XL, SL, C, D: Indicators of reinforced or extra load capacity (e.g., XL = Extra Load).

Brand, Model, and Tread Pattern Info

Brand and model names, along with specific tread pattern codes, help match replacements or compare reviews.

Special US Markings and Legal Considerations

Uniform Tyre Quality Grading (UTQG)

Most passenger tyres in the US display UTQG ratings for treadwear, traction, and temperature resistance. These are required by federal law but not on some winter and light truck tyres.

Sidewall Plies and Construction

Information such as “Tread 2 Plies Steel + 1 Ply Polyester” tells you about the internal construction, which can affect durability and ride comfort.

Recalls and Registration

DOT codes are essential for tyre registration and recall notifications. Always register your tyres with the manufacturer to receive safety alerts.

Common Mistakes

• Selecting tyres based only on size, ignoring load and speed ratings.
• Confusing maximum pressure with recommended inflation pressure.
• Overlooking tyre age by not checking the DOT code.
• Assuming all all-season tyres are suitable for winter conditions without looking for the 3PMSF symbol.
• Mixing different speed or load ratings on the same axle or vehicle.
• Ignoring special markings for light trucks (LT) or spares (T).
• Not registering tyres for recall alerts.

Quick Checklist

• Note the full size and markings from your current tyres before shopping.
• Confirm load and speed ratings meet or exceed your vehicle’s requirements.
• Check the DOT code for tyre age; aim for tyres less than 2 years old at purchase.
• Look for special markings if you need winter, run-flat, or extra load tyres.
• Use only the recommended inflation pressure from your vehicle, not the tyre sidewall.
• Register your new tyres for recall and safety updates.
• Ask your dealer to explain any unfamiliar sidewall codes before buying.

FAQ

How do I find my tyre size?

Check the sidewall for a code like 225/50R17, or consult your owner’s manual or the placard inside your driver’s door.

What if my tyre doesn’t match the original size?

Deviating from the recommended size can affect handling, speedometer accuracy, and safety. Only change sizes with professional guidance.

Why is the DOT code important?

The DOT code tells you when and where the tyre was made and is key for identifying recalls and tyre age.

How old is too old for a tyre?

Most experts recommend replacing tyres every 6 years, regardless of tread, and never using tyres over 10 years old.

What’s the difference between P, LT, and T in size markings?

P stands for passenger car, LT for light truck, and T for temporary spare. Always match your tyre type to your vehicle.

Can I mix load or speed ratings?

It’s not recommended. Mismatched ratings can compromise safety and performance. Use same ratings per axle, ideally per vehicle.

Is the maximum pressure on the sidewall the correct inflation pressure?

No, always use the pressure listed in your vehicle’s manual or door placard for correct inflation.

What do M+S and 3PMSF mean?

M+S means basic all-season capability; 3PMSF indicates the tyre meets true winter traction standards. Look for 3PMSF for snowy climates.

Conclusion

Tyre sidewall markings are more than just numbers and letters—they’re essential tools for safe, effective, and legal tyre selection in the US. By learning to read and interpret these codes, you can ensure your tyres match your vehicle, driving style, and local conditions. Before your next purchase or replacement, take a few minutes to inspect your tyres, write down all the sidewall information, and discuss any questions with your tyre dealer. Being informed not only protects your investment but also enhances safety for every mile ahead.