How to Read a Tire Sidewall

The anatomy of a sidewall

Everything on a tire sidewall exists because a regulator, a standards body or a liability lawyer required it. Nothing is decorative. The markings fall into four broad groups: dimensional (what size it is), capability (how much weight and speed it can handle), traceability (who made it, where, and when) and performance grading (how it compares to a reference tire).

Here is a typical passenger-car sidewall with the important zones labelled.

1. The size designation

The size string is the one everyone recognises. Take 225/45R17 94Y:

You can calculate overall diameter from the string alone:

Prefixes you may see

• P — P-metric, designed to US passenger standards. A P-metric tire fitted to a light truck must have its load capacity de-rated, typically by dividing by 1.10.
• LT — Light Truck. Heavier construction, higher inflation pressures, and load ranges (C, D, E) instead of a single load index.
• ST — Special Trailer. Stiffer sidewalls, not designed for steering or driving loads.
• No prefix — Euro-metric. Slightly higher load capacity than the equivalent P-metric size at the same inflation.
• M/C — motorcycle. Appears after the rim diameter, as in 120/70R17 M/C.

Imperial and off-road sizing

Powersports, agricultural and industrial tires often abandon the metric string entirely and use overall diameter × section width – rim diameter, all in inches. A 27×11R14 ATV tire is 27 inches tall, 11 inches wide, radial, on a 14-inch wheel. It is a more honest system because the first number tells you the actual height instead of requiring arithmetic. We cover the quirks in the ATV and UTV size guide, and bicycles have their own entirely separate mess covered in the ETRTO sizing guide.

2. Load index

The load index is a lookup code, not a unit. It expresses the maximum mass a single tire may carry at the maximum inflation pressure stamped on that sidewall. Drop the pressure and the real capacity drops with it — which is exactly why underinflation is the leading mechanical cause of tire failure.

3. Speed rating

The speed rating letter comes from a bench test: the tire runs on a 1.7 m steel drum under a specified load, with speed stepped up in 10 km/h increments held for 10 minutes each, until the target is reached and sustained without failure. It is a structural integrity rating — it says nothing about grip.

A higher rating is not free. To pass a faster test, engineers stiffen the sidewall and belt package and use compounds that stay stable at higher temperatures. Those same compounds tend to be harder when cold, so a W-rated summer tire in February is dramatically worse than a T-rated all-season — the letter that looks superior on paper is the one that lets go first on a cold morning. Match the rating your manufacturer specifies; exceed it only if you actually drive at those speeds.

4. UTQG: treadwear, traction, temperature

Uniform Tire Quality Grading is a US requirement, mandatory on passenger tires and absent on winter tires, LT tires and most imports sold outside the US. It reads like TREADWEAR 400 TRACTION A TEMPERATURE A.

• Treadwear is a relative index. A grade of 400 means the tire wore four times slower than the government reference tire (graded 100) over a 7,200-mile convoy test in Texas. Crucially, the manufacturer assigns the number itself based on its own test. It is reliable for comparing two tires from the same brand and nearly meaningless across brands.
• Traction (AA, A, B, C) measures straight-line wet braking coefficient on asphalt and concrete skid pads. It does not test dry grip, cornering or hydroplaning resistance.
• Temperature (A, B, C) measures resistance to heat generation at speed. C is the legal minimum; anything sold new today is normally A or B.

5. The DOT code — a tire's birth certificate

The DOT code, formally the Tire Identification Number (TIN), is the traceability marking. A full modern code looks like DOT B3 4T HR 3224:

Two practical warnings. First, regulations require the full code on only one sidewall; the other side may carry a partial code without the date. If you cannot find a date, check the other side. Second, a tire ages chemically whether or not it rolls. Antiozonants migrate out of the rubber, the polymer network stiffens, and sidewall flex cracking begins. Many manufacturers advise inspection from six years and replacement at ten regardless of tread depth. A “new old stock” bargain with a five-year-old date code is not a bargain.

Our DOT date code decoder converts a code to a build date and flags the age thresholds.

6. Seasonal and capability symbols

OE homologation codes

A small letter cluster near the size often indicates a vehicle-specific version: MO (Mercedes-Benz), AO/RO1 (Audi), * (BMW), N0–N7 (Porsche), TPC (General Motors), T0/T1 (Tesla). These are not marketing. The carmaker and tire maker co-developed the compound and casing for that platform's suspension tuning, and the “same” tire without the code can behave measurably differently. On a leased or warranty-covered car, matching the OE code is usually the right call.

What to actually check when buying

1. Open the driver's door and read the placard. The vehicle placard — not the tire currently fitted — is the authoritative source for size, load index, speed rating and cold inflation pressure. A previous owner may have fitted the wrong thing.
2. Match or exceed load index and speed rating. Going below either voids insurance in many jurisdictions and is genuinely dangerous.
3. Check the DOT date before it leaves the shop. Ask for tires less than 12 months old. This is a reasonable request and reputable shops will accommodate it.
4. Confirm the seasonal symbol you need. If you see snow, look for 3PMSF, not M+S.
5. Keep the set consistent. Mixing construction types (radial and bias) on the same axle is unsafe; mixing tread patterns front-to-rear degrades stability, especially on AWD systems that expect matched rolling circumferences.

Frequently asked questions