Alphanumeric code specifying tire sizes and limits
Automotive tires are described by several alphanumeric tire codes (in North American English) or tyre codes (in Commonwealth English), which are generally molded into the sidewall of the tire. These codes specify the dimensions of the tire and its key limitations, such as load-bearing ability and maximum speed. Sometimes the inner sidewall contains information not included on the outer sidewall, and vice versa.
The code has grown in complexity over the years, as is evident from the mix of SI and USC units, and ad-hoc extensions to lettering and numbering schemes. 
Most passenger car tires sizes are given using either the P Metric tire sizing system or the Metric tire sizing system (which is based on ISO standards but is not to be confused with the ISO metric system). Pickup trucks and SUVs use the Light Truck Numeric or Light Truck High Flotation system. Heavy trucks and commercial vehicles use another system altogether. 
ETRTO, TRA, and JATMA.
The European Tyre and Rim Technical Organisation (ETRTO) and the Tire and Rim Association (TRA), formerly known as The Tire and Rim Association of America, Inc., are two organizations that influence national tire standards. There exists also the Japan Automobile Tyre Manufacturers' Association, Inc. (JATMA). In practice, the standards of the three organizations have evolved together and are fairly interchangeable, though the load and inflation tables will give slightly different values for the same size tire.
In the United States, the Office of Vehicle Safety Compliance, a component of the Department of Transportation, is one of the agencies tasked to enforce the Federal Motor Vehicle Safety Standard (FMVSS). Canada has published tire regulations, such as the Motor Vehicle Tire Safety Regulations SOR 95-148.
Metric tire codes.
The metric tire code consists of a string of letters and numbers describing the dimensions of the tire, as follows:
Vehicle Class.
An optional letter (or letters) indicating the intended use or vehicle class for the tire:
Section Width.
A 3-digit number indicating the "nominal section width" of the tire in millimeters; the widest point from both outer edges (side wall to side wall). The tire surface that touches the road usually has a narrower width (called "tread width").
Slash.
A slash "/" character for character separation.
Aspect Ratio.
A 2- or 3-digit number indicating the "aspect ratio" of the sidewall height as a percentage of the nominal section width of the tire. If the information is omitted, it is assumed to be 82%. However, if the number is larger than 200, then this is the diameter of the entire tire in millimeters.
Construction.
An optional letter or two indicating construction of the fabric carcass of the tire:
Diameter.
A 1- or 2-digit number indicating the diameter, in inches, of the rim that the tires are designed to fit. There is the rare exception of metric-diameter tires, such as the use of the 390 size, which in this case would indicate a rim of 390 mm in diameter. Few tires are made to this size currently. The number may be longer where a half-inch size is used, for example many heavy transport trucks now use 22.5-inch tires.
Load Index and Load Range.
The load index on a passenger-car tire is a two- or three-digit numerical code used to cross-reference a load &amp; inflation table that will give the maximum load each tire can carry at a given pressure. 
The load index is sometimes used in conjunction with the load range, which appears elsewhere on the tire. It usually consists of two letters (usually LL for Light Load, SL for Standard Load, and XL for Extra Load). However, it may also be written out as "EXTRA LOAD." It may also be absent entirely, which indicates a Standard Load tire. 
These two codes allow one to look up the required cold inflation pressure to carry a given load, according to the load inflation charts or tables. The load tables are too large to include in this article, but may be found at tirepressure.org. Generally, tire codes with a preceding "P" shall reference the P Metric Tire Load Inflation Chart, while those without shall reference the Metric Tire Load Inflation Chart. 
Speed rating.
The speed symbol is made up of a single letter, or an A with one numeral. It is indicative of the maximum speed at which the tire can carry its rated load while ensuring that no part of the tire overheats when operating in steady-state conditions on smooth roads. 
Speed ratings of S and above have certain constraints that must be met in order to reach their maximum speeds. Namely, their operating pressures must be adjusted according to the table below.
Speed ratings with parentheses such as (W) and (Y) have maximum speeds set by the manufacturers. The load rating is often included within the parentheses, e.g. (86Y).
Prior to 1991, tire speed ratings were shown inside the tire size, before the "R" construction type. The available codes were SR (180 km/h, 112 mph), HR (210 km/h, 130 mph), VR (in excess of 210 km/h, 130 mph), and ZR (in excess of 240 km/h, 150 mph).
In many countries, the law requires that tires must be specified, and fitted, to exceed the maximum speed of the vehicle they are mounted on, with regards to their speed rating code (except for "temporary-use" spare tires). In some parts of the European Union, tires that are not fit for a car's or motorcycle's particular maximum speed are illegal to mount. The sole exception are M+S tires, where a warning sticker stating the allowed maximum speed must be placed within clear sight of the driver inside the vehicle. Some manufacturers will install a speed governor if a vehicle is ordered with tires rated below the vehicle's maximum speed. In some parts of the European Union, e.g. Germany, it is allowed to mount tires with a lower speed rating code if the car manufacturer specifies tires with a very high speed rating in the registration documents and the vehicle will not reach this speed based on insufficient power. In this case it is possible to calculate the appropriate speed rating with a formula.
Other codes.
Wear, Traction, and Temperature Grades.
The wear, traction, and temperature characteristics of passenger tires are displayed according to the Uniform Tire Quality Grading (UTQG) standard. 
DOT code.
The DOT code can be found immediately after the letters "DOT" on a tire sidewall. 
It is useful in identifying tires subject to product recall or at end of life due to age. It is mandated by the U.S. Department of Transportation but is used worldwide. 
The first three characters indicate the manufacturing plant and are assigned by the NHTSA (tires made before 2015 had only two characters).
Following the manufacturer's plant code, the next two digits within a DOT code signify the tire size. For instance, the size code "WC" might correspond to the tire size 205/55 R16. However, these size codes aren't standardized. Each tire manufacturer may assign these codes as they see fit, provided they maintain a consistent and explainable system within their coding. 
After the size code, the next sequence in a DOT number consists of three or four digits that designate the tire type. This coding, though optional, is widely used by tire manufacturers for internal tracking purposes, such as managing returns and quality control. The decision on how exactly to code the tire type lies with each manufacturer, leading to a variety of coding systems across the industry.
The last four numbers of the DOT marking on tires tell you the week (1 through 52) and year the tire was made. So, a tire with the last four digits of 0121 was made the first week of January 2021, 0221 is second week of January of 2021, and so on. 
E-mark.
All tires sold for road use in Europe after July 1997 must carry an E-mark. The mark itself is either an upper case "E" or lower case "e" – followed by a number in a circle or rectangle, followed by a further number. An (upper case) "E" indicates that the tire is certified to comply with the dimensional, performance and marking requirements of ECE Regulation 30. A (lower case) "e" indicates that the tire is certified to comply with the dimensional, performance and marking requirements of Directive 92/23/EEC. The number in the circle or rectangle denotes the country code of the government that granted the type approval. The last number outside the circle or rectangle is the number of the type approval certificate issued for that particular tire size and type.
Light truck (LT) tire codes.
Flotation tires keep vehicle tires floating above loosely packed dirt, minimizing soil disturbance in agricultural environments and maximizing vehicle stability on unpaved surfaces in construction environments. 
Light truck tires are indicated by the letters LT at the end instead of the beginning of the sequence, as follows:
As an example, if a tire size has two sets of numbers (6-12, 5.00-15, 11.2-24), then the first number (5.00-15) is the approximate width in inches, and the second number (5.00-15) is the rim diameter in inches.
If a tire size has three sets of numbers (15x6.00-6, 26x12.00-12, 31x15.50-15), then the first number (26x12.00-12) is the approximate tire diameter in inches, the second number (26x12.00-12) is the approximate width in inches, and the third number (26x12.00-12) is the rim diameter in inches.
Load range.
The load range letter on light-truck tires indicates their ply rating.
Wheel/rim widths.
To determine the allowable range of rim widths for a specific tire size, the TRA Yearbook or the manufacturer's guide should always be consulted for that specific tire – there is no rule of thumb. Running a tire on a rim size or type not approved by its manufacturer can result in tire failure and a loss of vehicle control.
Additional marks.
There are numerous other markings on a typical tire, these may include:
Tire geometry.
When referring to the purely geometrical data, a shortened form of the full notation is used. To take a common example, 195/55R16 would mean that the nominal width of the tire is approximately 195 mm at the widest point, the height of the side-wall of the tire is 55% of the width (107 mm in this example) and that the tire fits rims. The code gives a direct calculation of the theoretical diameter of the tire. For a size shown as "T/A_W" use (2×T×A/100) + (W×25.4) for a result in millimeters or (T*A/1270)+ W for a result in inches. Take the common example used above; (2×195×55/100)+(16×25.4) = 621 mm or (195×55/1270)+16 = 24.44 inches.
Less commonly used in the US and Europe (but often in Japan for example) is a notation that indicates the full tire diameter instead of the aspect ratio of the side-wall height. To take the same example, a 16-inch rim would have a diameter of 406 mm. Adding twice the tire height (2×107 mm) makes a total 620 mm tire diameter. Hence, a 195/55R16 tire might alternatively be labelled 195/620R16.
Whilst this is theoretically ambiguous, in practice these two notations may easily be distinguished because the height of the side-wall of an automotive tire is typically much less than the width. Hence when the height is expressed as a percentage of the width, it is almost always less than 100% (and certainly less than 200%). Conversely, vehicle tire diameters are always larger than 200 mm. Therefore, if the second number is more than 200, then it is almost certain the Japanese notation is being used – if it is less than 200 then the U.S./European notation is being used.
The diameters referred to above are the theoretical diameter of the tire. The actual diameter of a specific tire size can only be found in the TRA Yearbook or the manufacturer's data books. Note that the tire's cross-section and diameter are always specified when measured on a rim of a specified width; different widths will yield different tire dimensions.
Examples.
The tires on a BMW Mini Cooper might be labeled: "P195/55R16 85H"
The tires on a Hummer H1 might be labeled: "37X12.5R17LT"
Historical tire codes.
North America.
Prior to 1964, tires were all made to a 90% aspect ratio. Tire size was specified as the tire width in inches and the diameter in inches – for example, 6.50-15.
From 1965 to the early 1970s, tires were made to an 80% aspect ratio. Tire size was again specified by width in inches and diameter in inches. To differentiate from the earlier 90-ratio tires, the decimal point is usually omitted from the width – for example, 685-15 for a tire 6.85 inches wide.
Starting in 1972 tires were specified by load