Automotive tires are described by several alphanumeric tire codes (North American English spelling) or tyre codes (Commonwealth English spelling), 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.,Scope of the Association and Some of its History , The Tire and Rim Association. Retrieved 2010-01-04. 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.Care and Maintenance - Technical , Dunlop Tires. Retrieved 2010-01-05.
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).Laboratory Test Procedure for FMVSS 120 , National Highway Traffic and Safety Administration, United States Department of Transportation, April 10, 2000. Retrieved 2010-01-05. Canada has published tire regulations, such as the Motor Vehicle Tire Safety Regulations SOR 95-148.Motor Vehicle Tire Safety Regulations SOR 95-148, Canadian Legal Information Institute. Retrieved 2010-01-05.
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: P: Passenger car LT: Light truck C: Commercial trucks ST: Special trailer M/C: Motorcycle T: Temporary (restricted usage for "space-saver" spare wheels) A preceding "P" indicates that the tire loads are designed to TRA standards, while the absence of a letter indicates that the tire conforms to ETRTO standards.
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: B: bias belt (where the sidewalls are the same material as the tread, leading to a rigid ride) D: diagonal R: radial if omitted, it is a cross-ply tire The R in a radial tire may be preceded by an optional letter indicating the speed rating of the tire according to a deprecated naming system. This letter can be ignored as the actual speed rating is given by the letter appearing at the end of the metric tire code, following the load index.
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.A brief history of radial tires and the offbeat TRX system, Michelin TRX, Dr. Theo Netherlands. Retrieved 2010-01-05.Similarly, whilst most bicycle wheels have a diameter measured in inches, the 700mm wheel (=27.56") is increasingly common.
Load index and load range The tyre load index (LI) on a passenger-car tire is a two- or three-digit numerical code used to cross-reference a load and 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.
+Tyre load index (LI) tableLoad index (LI)kglbsLoad index (LI)kglbsLoad index (LI)kglbs0459910080017642001400030900146.510210182518192011450032000247.510510285018742021500033100348.71071038751929203155003420045011010490019842041600035300551.51141059252039205165003640065311710695020942061700037500754.51201079752149207175003860085612310810002205208180003970095812810910302271209185004080010601321101060233721019000419001161.513611110902403211195004300012631391121120246921220000441001365143113115025352132060045400146714811411802601214212004670015691521151215267921521800481001671157116125027562162240049400177316111712852833217230005070018751651181320291021823600520001977.517111913602998219243005360020801761201400308622025000551002182.518212114503197221257505680022851871221500330722226500584002387.51931231550341722327250601002490198124160035272242800061700Load index (LI)kglbsLoad index (LI)kglbsLoad index (LI)kglbs2592.520412516503638225290006390026952091261700374822630000661002797.521512717503858227307506780028100220128180039682283150069400291032271291850407922932500717003010623413019004189230335007390031109240131195042892313450076100321122471322000440923235500782643311525413320604541233365008046934118260134212046742343750082673351212671352180480623538750854293612527613622404938236400008818537128282137230050712374125090941381322911382360520323842500936963913630013924305357239437509645240140309140250055122404500099208411453201412575567724146250101964421503311422650584224247500104719431553421432725600824348750107475441603531442800617324450000110231451653641452900639324551500113538461703751463000661424653000116845471753861473075677924754500120152481803971483150684424856000123459491854081493250716524958000127868Load index (LI)kglbsLoad index (LI)kglbsLoad index (LI)kglbs501904191503350739025060000132300511954301513450761025161500135580522004411523550783025263000138890532064541533650805025365000143300542124671543750827025467000147710552184811553875854025569000152120562244941564000882025671000156530572305071574125909025773000160930582365201584250937025875000165340592435361594375965025977500170660602505511604500992026080000176400612575671614625102002618250018188062265584162475010500262850001873906327260016348751070026387500192900642806171645000110002649000019845065290639165515011400265925002039206630063916653001170026695000209440673076771675450120002679750021495068315694168560012300268100000220500693257161695800128002691030002273707033573917060001320027010600023373071345761171615013600271109000240345723557831726300139002721120002469607336580517365001430027311500025357574375827174670014800274118000260190Load index (LI)kglbsLoad index (LI)kglbsLoad index (LI)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
Speed rating The speed symbol or tyre speed index (SI) 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 ratingCodekm/hmphCodekm/hmphA153L12075A2106M13081A3159N14087A42012P15094A52516Q160100A63019R170106A73522S180112A84025T190118B5031U200124C6037H210130D6540V240149E7043Zover 240over 149F8050W270168G9056(W)over 270over 168J10062Y300186K11068(Y)over 300over 186
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 ratingMax speed S1 without pressure adjustment (km/h)Pressure adjustment for every 10 km/h over S1 (kPa)Maximum speed S2 without load reduction (km/h)Load reduction for every 10 km/h over S2 (kg)Maximum speed (km/h)S16051800%180T16071900%190H16062100%210V16062103%240W190102405%270Y220102705%300 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.
Date of manufacture