Every tire carries a dense block of letters and numbers on its sidewall. It looks like a part number, but almost every character means something specific: how big the tire is, what wheel it fits, how much weight it can carry, how fast it’s rated to go, and when it was made. Once you know the pattern, you can read most tires in a few seconds. This guide works through a typical marking from left to right, then covers the date code and the smaller symbols that are easy to miss.

Key points

  • A marking like 225/45 R17 94W gives width, sidewall proportion, construction, rim size, load index and speed rating.
  • The load index and speed letter are codes; you need a table to turn them into kilograms and km/h.
  • The last four digits of the DOT code are the manufacturing week and year.
  • The sizes, load index and speed rating your vehicle needs are on its door-jamb placard or in the owner’s manual.

The size marking, part by part

We’ll use P225/45 R17 94W XL as the example. In the UK, Europe and Australia you’ll usually see it without the P, as 225/45 R17 94W XL. The diagram shows each part; the sections below explain them.

Anatomy of a tire size marking The marking P225/45 R17 94W XL broken into its parts: P passenger type, 225 mm width, 45 percent aspect ratio, R radial, 17-inch rim, load index 94 meaning 670 kilograms per tire, speed rating W meaning 270 km/h, and XL extra load. P225/45R1794WXL Tire type P = passengernone = metricLT = light truck Section width 225 mm wide(8.86 in) Aspect ratio sidewall is 45%of width: 101.3 mm Construction R = radialZR = >240 km/h Rim diameter 17-inch wheel(431.8 mm) Load index 670 kg per tire(1,477 lb) max Speed rating W = 270 km/h(168 mph) Load version extra load:higher pressureand max load Overall diameter 634.3 mm · nominal values calculated from the marking
Anatomy of a size marking. Each coloured part of the marking is explained underneath. Values for this example are nominal, calculated from the marking itself.

Tire type: P, LT or nothing

A leading P marks a passenger tire built to the P-metric standard of the US Tire and Rim Association. With no letter in front, the size follows the European ETRTO standard, often called Euro-metric. The two are close and the dimensions are the same, but the load tables give slightly different load capacities for the same size, so check the load index rather than assuming they match. LT marks a light-truck tire, built for heavier loads and higher pressures; these follow different load rules, which is why our calculators only accept LT sizes where diameter is all that matters.

Section width: 225

The first number is the nominal section width in millimetres: the widest point of the tire’s cross-section, excluding raised lettering or rim protectors, measured when it’s mounted on its specified measuring rim. On a wider or narrower wheel, the actual width changes a little. Here, 225 mm is about 8.86 inches.

Aspect ratio: 45

The number after the slash is the aspect ratio: the sidewall height as a percentage of the section width. 45% of 225 mm gives a sidewall of 101.25 mm. A lower number means a shorter, stiffer sidewall. Because it’s a percentage, two tires with the same aspect ratio but different widths have different sidewall heights, which is a common source of confusion when comparing sizes.

Construction: R

R means radial construction, which covers almost every modern car tire. You may also see ZR in the size position, as in 225/45ZR17. ZR indicates a tire capable of more than 240 km/h (149 mph); the service description after the size then gives its exact rating. A few run-flat tires use RF in this position.

Rim diameter: 17

The rim diameter is in inches and must match the wheel exactly: a 17-inch tire only fits a 17-inch wheel. With the sidewall height and rim size you can work out the tire’s nominal overall diameter: 17 × 25.4 + 2 × 101.25 = 634.3 mm. That number drives speedometer accuracy, gearing and clearance, and it’s what the tire size calculator compares between sizes.

Load index: 94

The first part of the service description is the load index, a code for the maximum load one tire can carry at the pressure specified for that load. It isn’t a weight on its own: you look it up. Load index 94 means 670 kg (1,477 lb) per tire. A few neighbouring values show how it steps:

Selected load index values
Load indexMaximum load per tire
87545 kg (1,202 lb)
91615 kg (1,356 lb)
94670 kg (1,477 lb)
98750 kg (1,653 lb)
102850 kg (1,874 lb)
107975 kg (2,150 lb)
1121120 kg (2,469 lb)

The vehicle maker specifies a minimum load index, and replacement tires should meet it. The full table from 60 to 130 is on the sidewall decoder page.

Speed rating: W

The letter after the load index is the speed symbol: the maximum speed the tire is rated to sustain under the conditions of the standard test, when correctly inflated and not overloaded. W means 270 km/h (168 mph). The common ones run S (180 km/h, 112 mph), T (190, 118), H (210, 130), V (240, 149), W (270, 168) and Y (300, 186). Note that H sits between U and V, out of alphabetical order, a leftover from how the system grew. A speed rating describes the tire’s capability, not a safe speed for the road or the vehicle.

Load version: XL, RF or SL

XL (extra load) or RF (reinforced) after the service description means the tire is built to run at a higher inflation pressure than a standard-load tire of the same size, which gives it a higher maximum load. The extra capacity only exists at that higher pressure. Standard-load tires may be marked SL or carry no marking at all.

The DOT code and the tire’s age

Tires sold in the US carry a Tire Identification Number starting with DOT. It identifies the factory where the tire was made, includes size and manufacturer codes, and ends with the date of manufacture. Since 2000 the date is the last four digits: two for the week and two for the year. The code may be complete on only one side of the tire, so if you can’t find the date, look on the other sidewall, which might face inward on the car.

Reading the DOT date code An example Tire Identification Number, DOT 4B1V 2K7X 2319. The first group identifies the factory, the middle characters carry size and manufacturer codes, and the last four digits 2319 mean week 23 of 2019. DOT 4B1V 2K7X 2319 Factory code where it was made Size and maker codes manufacturer’s own coding 23 week 19 year 2019 made in the week of 3 June 2019 Four-digit date codes since 2000; three digits before. Characters before the date are illustrative.
The date is the last four digits. 2319 means the 23rd week of 2019. The characters before it identify the factory and carry the manufacturer’s own codes.

Tires made before 2000 used three digits: two for the week and one for the year within a decade, so 529 could be 1979, 1989 or 1999. Any tire with a three-digit date code is more than 25 years old.

Rubber ages whether or not the tire is used, faster with heat and sunlight. There’s no single age limit that applies to every tire, but some vehicle and tire makers publish guidance, and a tire that has spent years on a spare wheel or in storage deserves a professional inspection before it goes back into service. The decoder works out the exact age from the code.

Other markings worth knowing

  • Maximum load and maximum pressure. Small print such as “Max load 670 kg (1477 lbs) at 350 kPa (51 psi) max press” gives the tire’s limits. The maximum pressure is not the pressure to run; use the vehicle placard. Our tire pressure converter handles psi, bar and kPa.
  • M+S. Mud and snow: the tread pattern meets a geometric definition for mud and snow. It isn’t a test of winter performance.
  • Mountain and snowflake (3PMSF). A three-peak mountain with a snowflake shows the tire passed a standardised snow traction test.
  • UTQG grades. On most passenger tires sold in the US you’ll find a treadwear number and letter grades for traction and temperature, under the federal Uniform Tire Quality Grading rules. They’re comparative grades from the maker’s tests, best used to compare tires within one brand.
  • E-mark. An “E” in a circle followed by a number shows approval to UN ECE regulations; the number identifies the approving country.
  • TWI. A small triangle or “TWI” on the shoulder points to the tread wear indicator bars moulded into the grooves. When the tread is level with them, it’s down to 1.6 mm (about 2/32 in). Legal minimums vary by country.
  • Rotation arrows and OUTSIDE / INSIDE. Directional tires must turn the way the arrow points; asymmetric tires must be mounted with the side marked OUTSIDE facing out.

Reading a marking you’re unsure about

Light commercial (van) tires add a C after the rim size, as in 215/65 R16C, and often carry two load indices such as 109/107T: the first for single fitment, the second for dual wheels. Flotation sizes such as 33x12.50R15 work differently: the first number is the overall diameter in inches and the second is the width in inches. Older or specialist tires use other systems again. If the tire is for anything other than a passenger car or light truck, check the maker’s documentation.

Putting it together

So P225/45 R17 94W XL describes a passenger tire 225 mm wide, with a 101.3 mm sidewall, radial construction, for a 17-inch wheel, carrying up to 670 kg (1,477 lb) at its specified pressure, rated to 270 km/h (168 mph), in an extra-load construction. Its nominal overall diameter is 634.3 mm.

When you shop for replacements, start from the vehicle placard rather than the tires currently fitted, since a previous owner may have changed sizes. To decode any marking instantly, paste it into the tire sidewall decoder; to see how a different size would change diameter, speedometer reading and ride height, use the tire size calculator.

Sources