How to read a tire sidewall

Type the size and codes printed on your tire. Each marking is explained on its own line, with the load in pounds and kilograms, the speed rating, the week the tire was made and how old it is today.

Your sidewall

Markings on your tire

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Type what you can read, in any order, in upper or lower case. Nothing you type leaves your browser.

This explains what the markings mean. It does not tell you what your vehicle needs: check the tire placard (door sticker) and the owner's manual.

Where each marking is

Example markings. The layout differs from tire to tire, and the full DOT code is often on one side only.

How to read a tire size

Take P225/45R17 91V. P means a passenger car tire (LT means light truck). 225 is the section width in millimetres. 45 is the aspect ratio: the sidewall is 45% as tall as the tire is wide. R is radial construction (D is diagonal or bias ply, B is bias belted). 17 is the rim diameter in inches. The overall diameter follows from those numbers:

diameter (in) = rim + 2 × (width × aspect ÷ 100) ÷ 25.4

Worked example, 225/45R17: the sidewall is 225 × 45 ÷ 100 = 101.25 mm, or 3.99 in. The diameter is 17 + 2 × 3.99 = 24.97 in (634 mm). Inch sizes such as 33x12.50R15LT read directly: 33 in tall, 12.50 in wide, for a 15 in rim, LT for light truck.

Load index and speed symbol

The 91V after the size is the service description. 91 is the load index: 615 kg (1,356 lb) per tire. V is the speed symbol: 149 mph (240 km/h), the top speed at which the tire carries that load. Light truck tires often show two load indexes, such as 125/122S: the first is for single mounting, 1,650 kg (3,638 lb) per tire, and the second for dual mounting, 1,500 kg (3,307 lb) per tire. See the load index chart and the speed rating chart.

A Z inside the size (225/45ZR17) is the older way to mark a tire for over 240 km/h (149 mph). A speed symbol in brackets, such as (Y), is open-ended: over 300 km/h (186 mph). Before 1991 the speed was written inside the size: SR (180 km/h), HR (210 km/h), VR (over 210 km/h).

Every other marking explained

XL or Reinforced: extra load, a passenger tire built to carry more than the standard load (SL) version of the same size.
Load range (LT tires): a letter for strength. C is ply rating 6, D is 8, E is 10, F is 12. The ply rating is a strength rating, not the number of plies. More on LT vs P tires.
M+S (also M&S): the tread meets a geometry definition. There is no snow test behind it.
3PMSF, the three-peak mountain snowflake: the tire passed a snow traction test under UN R117. A passenger tire needs a snow traction index of at least 1.10 against a standard reference tire. The ice grip symbol goes further: a 3PMSF tire with an ice grip index of 1.18 or more.
Run flat: the run flat tire symbol is not standard. Tire makers use their own letters and set their own distance and speed limits.
UTQG (passenger tires only): treadwear, traction and temperature grades from 49 CFR 575.104. Treadwear is relative: 400 is graded to wear 4 times as well as 100 on the government test course. Traction AA, A, B and C rate straight-ahead wet braking. Temperature A, B and C show which stage of the test-wheel test the tire passes: A the 575 rpm stage, B the 500 rpm stage, C the FMVSS 109 minimum. See UTQG ratings.

Where the DOT code is

The Tire Identification Number starts with DOT and has 13 symbols under 49 CFR 574.5: a plant code of 3 symbols (2 on older tires), a manufacturer's code of 6 and a date code of 4. The full code is often on one side of the tire only, so if you find a short code, look at the other side. The date code may be laser etched.

The date code reads week, then year: 2423 means week 24 of 2023. Week 01 is the first full Sunday to Saturday week of the year, so week 24 of 2023 ran from Sunday, June 11 to Saturday, June 17, 2023. A three-digit code comes from before 2000 and does not say which decade. The tire date code page has more.

Questions

How do you read a tire size?

Width, aspect ratio, construction, rim. In 225/45R17, the tire is 225 mm wide, the sidewall is 45% of that (101.25 mm), R means radial and 17 is the rim diameter in inches. The overall diameter works out to 24.97 in (634 mm).

What does 3PMSF mean on a tire?

Three-peak mountain snowflake. The tire passed a snow traction test under UN R117: a passenger tire needs a snow traction index of at least 1.10 against the standard reference test tire. M+S on its own is a tread geometry marking with no snow test.

What is the run flat tire symbol?

There is no single standard symbol. Tire makers use their own letters on the sidewall, and set their own limits for distance and speed after a loss of air. Check the tire maker's information for your tire.

Where is the date code on a tire?

At the end of the DOT code. The last four digits are the week and year: 2423 is week 24 of 2023, Sunday, June 11 to Saturday, June 17. The full code is often on one side of the tire only.

What does 125/122 mean on a light truck tire?

Two load indexes. 125 is the load for single mounting, 1,650 kg (3,638 lb) per tire. 122 is the load for dual mounting, two tires side by side, 1,500 kg (3,307 lb) per tire.