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How to read tire sidewall codes

Every tire's sidewall encodes its width, aspect ratio, rim diameter, load index, speed rating, and a four-digit production date. Here's how to decode each marking and what to act on.

The string of numbers and letters on a tire's sidewall is the closest thing the tire industry has to a passport. It tells you the tire's intended size, the load it was engineered to carry, the speed it was tested to, and when it was built. Most of this information matters when you're replacing tires; one piece — the DOT date code — matters when you're inspecting them.

The size string: 235/40R18 95Y

Read left to right. The first number — 235 — is the tire's section width in millimeters, measured at its widest unloaded point. The second number — 40 — is the aspect ratio: the sidewall height as a percentage of the section width, in this case 40% of 235 mm, or 94 mm of sidewall. The R indicates radial construction (universal on US passenger tires since the 1980s; bias-ply still exists but you will not encounter it on a passenger car).

The 18 after the R is the rim diameter in inches. Note the inconsistency: section width and sidewall in mm, rim diameter in inches. That's a historical artifact of the tire size standard, not a typo.

Load index: 95

The load index is a coded number that maps to a maximum load rating per tire. 95 = 1,521 lbs / 690 kg. Lower numbers mean lower load capacity. The load index on a replacement tire must equal or exceed the load index printed on the door-jamb placard for your vehicle. Going below it is unsafe and is the most common error owners make when shopping on price alone. The full load-index table is published by the Tire and Rim Association and is reprinted in every tire shop's reference book.

Speed rating: Y

The single letter after the load index is the speed rating. It's the maximum sustained speed the tire was tested to without structural failure. A handful of common ratings:

For replacement: speed rating should match or exceed the placard. Mixing speed ratings across a single axle is not safe, and lower-rated tires on cars with ESC/ABS systems calibrated for a specific rating can change the behavior of those systems at speed.

DOT date code

Look for "DOT" followed by a string of characters ending in a four-digit number. The last four digits are the manufacturing date: first two are the week, last two are the year. 2418 means week 24 of 2018. This is the most important number on the sidewall after the size.

Tires age regardless of tread depth. Rubber compounds break down through oxidation and UV exposure. Most manufacturers recommend replacement at 6 years from the DOT date, with hard end-of-life at 10 years, even if the tread looks fine. This matters for low-mileage vehicles — a tire with 80% tread but a 2014 DOT date should be replaced.

Treadwear, traction, temperature: the UTQG

The UTQG stamp is three small markings, usually grouped together:

UTQG is a manufacturer self-rating, not an independent test result. Treat it as one data point among several. See our deeper guide on UTQG for what the numbers actually mean in independent testing.

Other markings to recognize

What to do with this information

When replacing tires, write down the full sidewall string of your current tires (or the placard string on the door jamb if your current tires deviate). The replacement should match or exceed every number on that string — size, load index, speed rating. Then check the DOT date on the new tires before mounting. Tires sit in retailer warehouses; "new" tires can easily be a year or more old. Anything more than 2 years from the DOT date should be questioned at the counter.

Frequently asked questions

What does the Z in 205/45ZR17 mean?
The Z is a speed-rating marker for tires rated above 149 mph (W, Y, or higher). On modern tires the speed rating is shown explicitly after the load index (e.g. 84W), so the Z prefix is redundant but still appears for legacy reasons.
Is a higher UTQG treadwear number always better?
For long-life touring tires, yes — higher treadwear maps to longer wear life. For performance summer tires, higher treadwear typically means a harder compound with less peak grip. The right number depends on what you're trying to optimize.
What does ET45 mean on a wheel?
ET stands for the German Einpresstiefe (insertion depth). It's the wheel's offset in millimeters — the distance between the wheel's mounting face and its centerline. Positive offset (ET45) pushes the wheel inward; negative offset pushes it outward. Replacement wheels should match the OEM offset within a few mm to preserve suspension geometry.
Where is the DOT code printed?
On the inner sidewall, usually near the bead. It starts with the letters DOT and ends with a four-digit number (WWYY). One side of the tire is often stamped with the partial code only — flip the tire or check both sides if you don't see the full 4-digit string.

Sources

By Mark Bishop · Updated 2026-04-30.