Automobile

What the Numbers on Your Tire Sidewall Actually Mean

Automobile Editorial TeamStaff Writer · 6 min read · Mar 20, 2025

What the Numbers on Your Tire Sidewall Actually Mean

In This Article

Those codes stamped on your tires reveal load ratings, speed limits, and age. Here's how to read them before they become a safety issue.

Key Takeaways

A standard tire code like P215/65R17 93H packs six distinct data points into one string.
The speed rating letter indicates the maximum sustained speed the tire is designed to handle safely.
Load index numbers follow a standardized chart — they are not the actual weight in pounds.
The DOT date code reveals the tire's manufacturing week and year; tires older than six years warrant inspection.
Mismatched or worn tires are a leading contributor to vehicle crashes on US roads.

Breaking Down the Main Code

Take a tire stamped with P215/65R17 93H. Each segment means something specific:

P — Tire class: passenger vehicle
215 — Section width in millimeters (measured at the widest point)
65 — Aspect ratio: the sidewall height is 65% of the section width
R — Construction type: radial (the standard for modern passenger tires)
17 — Rim diameter in inches
93H — Load index (93) and speed rating (H)

Understanding this sequence lets you confirm that a replacement tire matches what your vehicle was engineered to use. Substituting a tire with a lower load index or smaller rim diameter than specified by the manufacturer can create a genuine safety hazard.

11,000+

Tire-related crashes per year in the US

The NHTSA estimates that tire-related factors contribute to more than 11,000 crashes annually on American roads.

6 years

Recommended tire inspection threshold by age

Most major tire manufacturers recommend that tires six years or older receive a professional inspection regardless of remaining tread depth.

~25%

Vehicles with at least one significantly underinflated tire

According to NHTSA data from a national roadside survey, roughly one in four passenger vehicles had at least one tire significantly underinflated — a condition that worsens load-rating and handling performance.

Load Index and Speed Rating Explained

The load index is a number between roughly 70 and 126 for passenger tires. It doesn't directly state a weight — it refers to a standardized chart. A load index of 93 corresponds to a maximum load of 1,433 lbs per tire. Multiply by four tires and you get the tire system's total capacity. Your vehicle's required load index is listed in the owner's manual and often on a placard inside the driver's door jamb.

The speed rating letter follows the load index. Common ratings include:

S — up to 112 mph
T — up to 118 mph
H — up to 130 mph
V — up to 149 mph
W — up to 168 mph

These are maximum sustained speed thresholds under load, not everyday driving targets. Installing a tire with a lower speed rating than your vehicle's specification is generally not recommended. For context on how safety ratings work across the auto industry more broadly, see our guide to NHTSA and IIHS safety scores.

Check Your Door Jamb First

Your vehicle's recommended tire size, load index, and inflation pressure are printed on a placard inside the driver's door jamb. This placard — not the tire itself — is your authoritative reference for what specification to match when purchasing replacements. Always cross-reference it before buying.

Reading the DOT Code and Tire Age

Separate from the size code, every tire sold in the US carries a DOT (Department of Transportation) code. It begins with the letters DOT and ends with a four-digit date stamp. That final group is the one that matters most to safety-conscious drivers.

For example, a code ending in 1821 means the tire was manufactured in the 18th week of 2021. Rubber degrades over time regardless of mileage — heat, UV exposure, and oxidation all accelerate the process invisibly. Cracks may develop inside the tire structure before they're visible on the surface.

Tread Depth and Age Are Separate Issues

A tire can pass a visual tread-depth check and still be compromised by age-related rubber degradation. The DOT date code is the only reliable way to assess a tire's age. If you're buying used tires or inspecting a vehicle that's been in storage, always locate and decode the DOT stamp before making decisions about continued use.

Tread depth tells you about wear, but the DOT date code tells you about age-related degradation — and both matter independently. A tire with plenty of tread but a date code from 2015 may warrant professional inspection or replacement.

Why These Numbers Are a Safety Issue

The NHTSA has identified tire failure as a contributing factor in thousands of crashes annually on US roads. The most common risk factors include underinflation, overloading beyond the load index, and operating tires past their useful service life.

Checking your sidewall codes takes less than two minutes. Confirm that your tires match the specification on your door-jamb placard, note the DOT date, and factor age into your maintenance timeline — not just tread wear. If you're uncertain whether a tire is appropriate for your vehicle or conditions, a licensed tire professional is the right resource. This article provides general educational information and is not a substitute for professional inspection of your specific vehicle.

Frequently Asked Questions

What does the P mean at the start of a tire code?

P stands for 'passenger vehicle.' You may also see LT (light truck) or ST (special trailer). These prefixes indicate what category of vehicle the tire is engineered for and affect load-carrying ratings.

How do I find out how old my tires are?

Look for the DOT code on the sidewall. The last four digits are the key — the first two indicate the week of manufacture and the last two indicate the year. '2319' means the 23rd week of 2019.

What happens if I exceed my tire's speed rating?

Exceeding a tire's speed rating can cause heat buildup that degrades the rubber structure, increasing the risk of a blowout. Speed ratings are not suggested limits — they are engineering thresholds.

Can I put a tire with a higher load index than required on my car?

Generally yes — a higher load index provides a safety margin above the minimum. However, always verify compatibility with your vehicle's owner manual and consult a qualified tire professional before substituting specifications.

When should tires be replaced based on age alone?

Most tire manufacturers and safety organizations recommend replacing tires after six years regardless of tread depth, and treating any tire over ten years old as end-of-life. Check your vehicle and tire manufacturer guidelines for specific recommendations.

Automobile Editorial Team

Staff Writer

Automobile Editorial Team is the collective byline for our editorial team and contributor network. Articles published under this byline or an editorial pen name are researched, written, and reviewed according to our editorial standards for clarity, consistency, and independence before publication.