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., 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:
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.
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.
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 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.
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