Tire code or Tyre code - Automobile tires are described by an alphanumeric code , 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 metric and imperial 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.
Explanation of tire codes
Two major standardizing organizations are the European Tyre and Rim Technical Organization (ETRTO) and the U. S. Tire and Rim Association (T&RA).
The ISO Metric tire code consists of a string of letters and numbers, as follows:
- An optional letter (or letters) indicating the intended use or vehicle class for the tire:-
- P : Passenger Car
- LT : Light Truck
- ST : Special Trailer
- T : Temporary (restricted usage for "Space-Saver" spare wheels)
- 3 digit number : The "nominal section width" of the tire in millimeters; the widest point from both outer edges.
- / : Slash character
- 2 or 3 digit number : The "aspect ratio" of the sidewall height to the total width of the tire, as a percentage. If this is missing, it is assumed to be 82%. If the number is larger than 200, then this is the diameter of the entire tire in millimeters.
- An optional letter 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 missing, then it is a cross ply tire
- 2 digit number : Diameter in inches of the wheel that the tire is designed to fit
- 2 or 3 digit number : Load index; see table below
- 1 or 2 digit/letter combo : Speed rating; see table below
- Additional marks: See subheading below.
Light Trucks
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:
- The tire diameter is given for High Flotation tires and omitted from Numeric tires.
- 2 digit number : The diameter of the tire in inches.
- x : Separator character.
- 3 or 4 digit number : The section width (cross section) of the tire in inches. If the tire diameter is not given, section widths ending in zero (e.g., 7.00 or 10.50) indicate the aspect ratio is 92%, while section widths not ending in zero (e.g., 7.05 or 10.55) indicate the aspect ratio is 82%.
- Construction of the fabric of the tire:
- B : bias belt
- D : diagonal
- R : radial
- 2 digit number : Diameter in inches of the wheel rim that this tire is designed to fit.
- LT : Designates that this is a Light Truck tire.
- Load index and speed rating are not required for this type of tire but may be provided by the manufacturer.
- 2 or 3 digit number : Load index; see table below.
- 1 or 2 digit/letter combo : Speed rating; see table below.
- Additional marks: See subheading below.
Load Range
Load range on light truck tires indicates ply rating and load pressure (PSI).
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.
Speed rating
The code is made up of one or two letters, or one letter and one number. It indicates the maximum permitted speed that the tire can sustain for a ten minute endurance without being in danger.
Prior to 1991, tire speed ratings were shown inside the tire size, before the "R" character. The available codes were SR (112 mph, 180 km/h), HR (130 mph, 210 km/h), VR (in excess of 130 mph, 210 km/h) and ZR (in excess of 149 mph, 240 km/h). The ZR code is still in common use, often combined with a W or Y speed code.
Tires with a speed rating higher than 186 mph (300 km/h) are indicated by a Y in parenthesis. The load rating is often included within the parenthesis, 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 the European community, 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.
If a tire is replaced with a lower speed rating than originally specified by the vehicle manufacturer, then this may render the vehicle insurance invalid.
Additional marks
There are numerous other markings on a typical tire, these may include:
- M+S , or M&S : Mud and Snow; all-weather tires, with above-average traction in muddy or very light snowy conditions, and for low ambient temperatures. Not a winter tire unless it has the snowflake and mountain symbol. Spike tires have an additional letter, "E" (M+SE).
- BSW : Black SideWall
- WSW : White SideWall
- OWL : Outline White Lettering
- E4 : Tire approved according ECE-regulations, the number indicating the country of approval.
- 030908 : Approval number of the tire
- DOT code : All tires for use in the USA have the DOT code, as required by the Department of Transportation (DOT). It specifies the company, factory, mold, batch, and date of production (2 digits for week of the year plus 2 digits for year; or 2 digits for week of the year plus 1 digit for year for tires made prior to 2000). Although not law, tire manufacturers do not suggest using a "new" tire that has been sitting on the shelf for more than 6 years.
- TL : Tubeless
- TT : Tube-type, tire must be used with an inner-tube
- Made in ... : Country of production
- C : Commercial; tires for light trucks (Example: 185 R14 C)
- B : Bias belted; tires for motorcycles (Example: 150/70 B 17 69 H)—diagonal construction with belt added under the tread
- SFI , or Inner : Side Facing Inwards; inside of asymmetric tires
- SFO , or Outer : Side Facing Outwards; outside of asymmetric tires
- TWI : Tread Wear Indicator; a device, such as a triangle or a small Michelin Man icon, located where the tread meets the sidewall. It indicates the location of the raised wear bars in between the tire tread channels.
- SL : Standard Load; tire for normal usage and loads
- XL : eXtra Load; tire for vehicles of heavier standard weights
- RF : Reinforced tires
- Arrows : Some tread designs are "directional", and designed to perform better when driven in a specific direction (clockwise or counter-clockwise). Such tires will have an arrow showing which way the tire should rotate when the vehicle is moving forwards. It is important not to put a "clockwise" tire on the left hand side of the car or a "counter-clockwise" tire on the right side.
- M0 : Original tires for Mercedes-Benz
- N* : Original tires for Porsche
- "Star" : Original tires for BMW
- To facilitate proper balancing, most high performance tire manufacturers place laso red (uniformity) and yellow (weight) dots - marks on the sidewalls of its tires to enable the best possible match-mounting of the tire/wheel assembly.
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
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