Tire code

Automotive tires are described by an alphanumeric tire code (in North American English) or tyre code (in Commonwealth English), which is generally molded into the sidewall of the tire. This code specifies the dimensions of the tire, and some of 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. New automotive tires frequently have ratings for traction, treadwear, and temperature resistance (collectively known as The Uniform Tire Quality Grade (UTQG) ratings).
Most tires sizes are given using the ISO metric sizing system. However, some pickup trucks and SUVs use the Light Truck Numeric or Light Truck High Flotation system.
National technical standards regulations.
DOT code.
The DOT code is an alphanumeric character sequence molded into the sidewall of the tire and allows the identification of the tire and its age. The code is mandated by the U.S. Department of Transportation but is used worldwide. The DOT code is also useful in identifying tires subject to product recall or at end of life due to age. Since 1st February 2021 UK vehicle Regulations do not permit the use of tyres over 10 years old on the front steered axle(s) of heavy goods vehicles, buses and coaches. The ban also applies to all tyres in single configuration on minibuses. In addition, it is a requirement for all tyres on these vehicles to display a legible date code.
ETRTO and TRA.
The European Tyre and Rim Technical Organisation (ETRTO) and the Tire and Rim Association (TRA) are two organizations that influence national tire standards. The objective of the ETRTO include aligning national tire and rim standards in Europe. The Tire and Rim Association, formerly known as The Tire and Rim Association of America, Inc., is an American trade organization which standardizes technical standards. 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.
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.
Explanation of tire codes.
The ISO Metric tire code consists of a string of letters and numbers, as follows:
P indicates that the tire is engineered to TRA standards, and absence of a letter indicates that the tire is engineered to ETRTO standards. In practice, the standards of the two organizations have evolved together and are fairly interchangeable, but not fully, since the load index will be different for the same size tire.
Flotation sizes.
Some light-truck tires follow the Light Truck Numeric or Light Truck High Flotation systems, 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 height 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.
Load index.
The load index on a passenger-car tire is a numerical code stipulating the maximum load (mass, or weight) each tire can carry. For load range "B" tires, ETRTO (ISO-Metric) standards specify the load index rating at an inflation pressure of (table below), while P-Metric standards measure the load capacity at an inflation pressure of . The two standards vary slightly with the capacity required for different inflation pressures.
While all ETRTO tires of the same load index will have the same maximum load, P-metric tires with the same load index may have different load capacities depending on the tire size. The TRA Inflation Tables must always be consulted when comparing the load capacity of P-Metric tires; the load index alone is not sufficient. An example: a P205/50R15 standard load tire has a load index of 84 and a load rating of at . A P215/50R13 with the same load index of 84 only has a load rating of , also at .
ETRTO produces a Standards Manual (current edition 2010), which contains a number of specifications and tables. The load index table (2010 page G7) lists the load index from to (although it appears to relate to an inflation pressure of it doesn't specify, but see load inflation table). The load inflation table references the load index to inflation pressures between and at intervals which is too large to be included here.
Some of the older letter-code load-range ratings for Light Truck Tires can be found in a chart published by the Goodyear Tire & Rubber Company. For example:
Speed rating.
The speed symbol is made up of a single letter or an A with one numeral. It indicates the maximum speed at which the tire can carry a load corresponding to its load index. The testing method consists of pressing the tire against a large diameter metal drum to reflect its appropriate load, and run at ever increasing speeds in 10 km/h (6.2 mph) steps in 10 minute increments until the tire's required speed has been met.
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).
Tires with a speed rating higher than 300 km/h (186 mph) are indicated by a Y in parentheses. The load rating is often included within the parentheses, e.g. (86Y).
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.
Wear rating.
The treadwear is a 3 digit comparative rating. It is part of Uniform Tire Quality Grading standard.
Metric to USC tire conversion chart.
R15
215/75/15 27.7"x 8.5"
225/70/15 27.4"x 8.9"
225/75/15 28.3"x 8.9"
235/75/15 29.0"x 9.3"
245/75/15 29.5"x 9.6"
255/75/15 30.0"x 10.0"
265/75/15 30.6"x 10.4"
R16
205/85/16 29.7"x 8.1"
215/75/16 28.7"x 8.5"
225/70/16 28.4"x 8.9"
225/75/16 29.2"x 8.9"
235/70/16 29.0"x 9.3"
235/85/16 31.7"x 9.3"
245/70/16 29.5"x 9.6"
245/75/16 30.5"x 9.6"
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 tirethere 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 wheels. 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 wheel 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 usedif 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 wheel 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 rating, using a letter code. In practice, a higher load rating tire was also a wider tire. In this system a tire had a letter, optionally followed by "R" for radial tires, followed by the aspect ratio, a dash and the diameter – C78-15 or CR78-15 for bias and radial, respectively. Each diameter of wheel had a separate sequence of load ratings; thus, a C78-14 and a C78-15 are not the same width. An aspect ratio of 78% was typical for letter-sized tires, although 70% was also common and lower profiles down to 50% were occasionally seen.