Automobile tires are described by an alphanumeric tire code (in American English and Canadian English) or tyre code (in British English, Australian English and others), 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.
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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.[1] 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.[2] 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).[3] Canada has published tire regulations, such as the Motor Vehicle Tire Safety Regulations SOR 95-148.[4]
The ISO Metric tire code consists of a string of letters and numbers, as follows:
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 Load Range Letter on light truck tires indicates their ply rating.[7]
| Load Range | Ply Rating |
|---|---|
| A | 2 |
| B | 4 |
| C | 6 |
| D | 8 |
| E | 10 |
| F | 12 |
| G | 14 |
| H | 16 |
| J | 18 |
| L | 20 |
| M | 22 |
| N | 24 |
There is a common misconception that there is a specific equivalence between a tire's Load Range (or ply rating) and its inflation pressure at which it achieves its maximum load.[8] In reality, tires of the same Load Index may require dramatically different pressures to achieve their published load ratings.
This can be seen by consulting the standards published every year by the industry standards group, the Tire and Rim Association. As an example, their 2010 Year Book shows that Load Range C tires may require 35 psi (240 kPa) or 55 psi (380 kPa), Load Range D tires may require 50 psi (340 kPa) or 65 psi (450 kPa), and Load Range E tires may require 65 psi (450 kPa) or 80 psi (550 kPa) to achieve their Load Index and Maximum Loads.[9] This is also the case in the tables published by major tire makers, who do follow these TRA standards and have multiple inflation pressures for the same Load Range in the tires they sell. [10]
It is essential to consult the guides like those just mentioned when making tire substitutions, and to read exactly what is imprinted on the sidewalls of tires. A different size of tire with the same Load Range may require a higher inflation pressure, and may fail in use if under-inflated.
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 36 psi (250 kPa) (table Below) while P-Metric standards measure the load capacity at an inflation pressure of 35 psi (240 kPa). The two standards vary slightly with the capacity required for different inflation pressures.[11]
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 505 kg (1,113 lb) at 35 psi (240 kPa). A P215/50R13 with the same load index of 84 only has a load rating of 495 kg (1,091 lb), also at 35 psi (240 kPa).[12]
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 0–45 kg (0–99 lb) to 279–136,000 kg (620–300,000 lb) (although it appears to relate to an inflation pressure of 42 psi (290 kPa) it doesn't specify, but see Load Inflation Table). [13] The Load Inflation Table references the load index to inflation pressures between 22 psi (150 kPa) and 42 psi (290 kPa) at 1 psi (6.9 kPa) intervals which is too large to be included here. [14]
| Code | Weight | Code | Weight | Code | Weight | Code | Weight | ||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 60 | 250 kg (550 lb) | 80 | 450 kg (990 lb) | 100 | 800 kg (1,800 lb) | 120 | 1,400 kg (3,100 lb) | ||||
| 61 | 257 kg (570 lb) | 81 | 462 kg (1,020 lb) | 101 | 825 kg (1,820 lb) | 121 | 1,450 kg (3,200 lb) | ||||
| 62 | 265 kg (580 lb) | 82 | 475 kg (1,050 lb) | 102 | 850 kg (1,900 lb) | 122 | 1,500 kg (3,300 lb) | ||||
| 63 | 272 kg (600 lb) | 83 | 487 kg (1,070 lb) | 103 | 875 kg (1,930 lb) | 123 | 1,550 kg (3,400 lb) | ||||
| 64 | 280 kg (620 lb) | 84 | 500 kg (1,100 lb) | 104 | 900 kg (2,000 lb) | 124 | 1,600 kg (3,500 lb) | ||||
| 65 | 290 kg (640 lb) | 85 | 515 kg (1,140 lb) | 105 | 925 kg (2,040 lb) | 125 | 1,650 kg (3,600 lb) | ||||
| 66 | 300 kg (660 lb) | 86 | 530 kg (1,200 lb) | 106 | 950 kg (2,100 lb) | ||||||
| 67 | 307 kg (680 lb) | 87 | 545 kg (1,200 lb) | 107 | 975 kg (2,150 lb) | ||||||
| 68 | 315 kg (690 lb) | 88 | 560 kg (1,200 lb) | 108 | 1,000 kg (2,200 lb) | ||||||
| 69 | 325 kg (720 lb) | 89 | 580 kg (1,300 lb) | 109 | 1,030 kg (2,300 lb) | ||||||
| 70 | 335 kg (740 lb) | 90 | 600 kg (1,300 lb) | 110 | 1,060 kg (2,300 lb) | ||||||
| 71 | 345 kg (760 lb) | 91 | 615 kg (1,360 lb) | 111 | 1,090 kg (2,400 lb) | ||||||
| 72 | 355 kg (780 lb) | 92 | 630 kg (1,400 lb) | 112 | 1,120 kg (2,500 lb) | ||||||
| 73 | 365 kg (800 lb) | 93 | 650 kg (1,400 lb) | 113 | 1,150 kg (2,500 lb) | ||||||
| 74 | 375 kg (830 lb) | 94 | 670 kg (1,500 lb) | 114 | 1,180 kg (2,600 lb) | ||||||
| 75 | 387 kg (850 lb) | 95 | 690 kg (1,500 lb) | 115 | 1,215 kg (2,680 lb) | ||||||
| 76 | 400 kg (880 lb) | 96 | 710 kg (1,600 lb) | 116 | 1,250 kg (2,800 lb) | ||||||
| 77 | 412 kg (910 lb) | 97 | 730 kg (1,600 lb) | 117 | 1,285 kg (2,830 lb) | ||||||
| 78 | 425 kg (940 lb) | 98 | 750 kg (1,700 lb) | 118 | 1,320 kg (2,900 lb) | ||||||
| 79 | 437 kg (960 lb) | 99 | 775 kg (1,710 lb) | 119 | 1,360 kg (3,000 lb) |
The speed symbol is made up of a single letter or an A with one number. It indicates the maximum speed at which the tire can carry a load corresponding to its Load Index. [16]
| Code | mph | km/h | Code | mph | km/h | |
|---|---|---|---|---|---|---|
| A1 | 3 | 5 | L | 75 | 120 | |
| A2 | 6 | 10 | M | 81 | 130 | |
| A3 | 9 | 15 | N | 87 | 140 | |
| A4 | 12 | 20 | P | 94 | 150 | |
| A5 | 16 | 25 | Q | 100 | 160 | |
| A6 | 19 | 30 | R | 106 | 170 | |
| A7 | 22 | 35 | S | 112 | 180 | |
| A8 | 25 | 40 | T | 118 | 190 | |
| B | 31 | 50 | U | 124 | 200 | |
| C | 37 | 60 | H | 130 | 210 | |
| D | 40 | 65 | V | 149 | 240 | |
| E | 43 | 70 | Z | over 149 | over 240 | |
| F | 50 | 80 | W | 168 | 270 | |
| G | 56 | 90 | (W) | over 168 | over 270 | |
| J | 62 | 100 | Y | 186 | 300 | |
| K | 68 | 110 | (Y) | over 186 | over 300 |
Prior to 1991, tire speed ratings were shown inside the tire size, before the "R" construction type. 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).
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 some parts of 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.
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.[18] 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.
There are numerous other markings on a typical tire, these may include:
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 16-inch-diameter (410 mm) 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 USA and Europe (but often in Japan for example) is a notation that indicates the full tire diameter instead 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 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.[22] 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.
The tires on a BMW Mini Cooper might be labeled: P195/55R16 85H
The tires on a Hummer H1 might be labeled: 37X12.5R17LT
The numeric codes on tractor tires since 1955 have required either two or three numbers: W-D or H/W-D where 'W' is the width of the tire in inches, D is the diameter of the rim in inches and H (if provided) is the percentage height of the tire. Hence, 5.00-15 is a tire that will fit a 15-inch-diameter (380 mm) rim and is 5 in (130 mm) wide but of indeterminate height. 25/5-16 is a tire that has a 5 in (130 mm) width, fits a 16-inch-diameter (410 mm) rim and whose height is approximately 25 in (640 mm).[23]