Tire code

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 (or moulded) 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 S.I. and English 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

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]

Explanation of tire codes

Tire identification diagram

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.[5]

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.[9]

Load range

The Load Range Letter on light-truck tires indicates their ply rating.[10]

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

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 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 (615–299,829 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.[12]

Standard load table (extract from ETRTO standards manual – 2010 page G7
ref 42 psi (290 kPa))[13]
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 (567 lb) 81 462 kg (1,019 lb) 101 825 kg (1,819 lb) 121 1,450 kg (3,200 lb)
62 265 kg (584 lb) 82 475 kg (1,047 lb) 102 850 kg (1,870 lb) 122 1,500 kg (3,300 lb)
63 272 kg (600 lb) 83 487 kg (1,074 lb) 103 875 kg (1,929 lb) 123 1,550 kg (3,420 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 (1,135 lb) 105 925 kg (2,039 lb) 125 1,650 kg (3,640 lb)
66 300 kg (660 lb) 86 530 kg (1,170 lb) 106 950 kg (2,090 lb) 126 1,650 kg (3,638 lb)
67 307 kg (677 lb) 87 545 kg (1,202 lb) 107 975 kg (2,150 lb) 127 1,750 kg (3,858 lb)
68 315 kg (694 lb) 88 560 kg (1,230 lb) 108 1,000 kg (2,200 lb) 128 1,800 kg (3,968 lb)
69 325 kg (717 lb) 89 580 kg (1,280 lb) 109 1,030 kg (2,270 lb) 129 1,850 kg (4,079 lb)
70 335 kg (739 lb) 90 600 kg (1,300 lb) 110 1,060 kg (2,340 lb) 130 1,900 kg (4,189 lb)
71 345 kg (761 lb) 91 615 kg (1,356 lb) 111 1,090 kg (2,400 lb) 131 1,950 kg (4,289 lb)
72 355 kg (783 lb) 92 630 kg (1,390 lb) 112 1,120 kg (2,470 lb) 132 2,000 kg (4,409 lb)
73 365 kg (805 lb) 93 650 kg (1,430 lb) 113 1,150 kg (2,540 lb) 133 2,065 kg (4,541 lb)
74 375 kg (827 lb) 94 670 kg (1,480 lb) 114 1,180 kg (2,600 lb) 134 2,125 kg (4,674 lb)
75 387 kg (853 lb) 95 690 kg (1,520 lb) 115 1,215 kg (2,679 lb) 135 2,185 kg (4,806 lb)
76 400 kg (880 lb) 96 710 kg (1,570 lb) 116 1,250 kg (2,760 lb) 136 2,245 kg (4,938 lb)
77 412 kg (908 lb) 97 730 kg (1,610 lb) 117 1,285 kg (2,833 lb) 137 2,305 kg (5,071 lb)
78 425 kg (937 lb) 98 750 kg (1,650 lb) 118 1,320 kg (2,910 lb) 138 2,365 kg (5,203 lb)
79 437 kg (963 lb) 99 775 kg (1,709 lb) 119 1,360 kg (3,000 lb) 139 2,435 kg (5,357 lb)

Some of the older letter-code load-range ratings for Light Truck Tires can be found in a chart[14] published by the Goodyear Tire & Rubber Company. For example:

Tire size Weight (lb) @lb/in2 Weight (lb) @lb/in2
LR Code D E
LT215/85R16 2335 @65 2680 @80
LT225/75R16 2335 @65 2680 @80
LT235/85R16 2623 @65 3042 @80
LT245/75R16 2623 @65 3042 @80

Speed rating

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.[13]

Speed rating[13]
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 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.[15] In this case it is possible to calculate the appropriate speed rating with a formula.[16]

Metric to Imperial 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"

265/70/17 30.6"x 10.

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 tire—there is no rule of thumb.[17] 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 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 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 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.

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.[23]

From 1965 to the early 70's, 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. 78% aspect ratio was typical for letter sized tires, although 70% was also common and lower profiles down to 50% were occasionally seen.[24]

See also

References

  1. About us, European Tyre and Rim Technical Organisation. Retrieved 2010-01-05.
  2. Scope of the Association and Some of its History, The Tire and Rim Association. Retrieved 2010-01-04.
  3. Laboratory Test Procedure for FMVSS 120, National Highway Traffic and Safety Administration, United States Department of Transportation, April 10, 2000. Retrieved 2010-01-05.
  4. Motor Vehicle Tire Safety Regulations SOR 95-148, Canadian Legal Information Institute. Retrieved 2010-01-05.
  5. Care and Maintenance - Technical, Dunlop Tires. Retrieved 2010-01-05.
  6. A brief history of radial tires and the offbeat TRX system, Michelin TRX, Dr. Theo Netherlands. Retrieved 2010-01-05.
  7. Similarly, whilst most bicycle wheels have a diameter measured in inches, the 700mm wheel (=27.56") is increasingly common.
  8. "FAQ's". Retrieved 2013-06-27.
  9. "2010 Year Book",The Tire and Rim Association, Inc.,Preface p XVII
  10. Guidelines for the Application on Load and Inflation Tables, Toyo Tires. Retrieved 2010-01-05.
  11. 1 2 Guidelines for the Application on Load and Inflation Tables, Toyo Tires. Retrieved 2010-01-05.
  12. 1 2 3 4 European Tyre and Rim Technical Organisation Standards Manual 2010 G6,
  13. "Goodyear Truck Tires Service Manual", Section 11, Retreading, p107
  14. "Council Directive 92/23/EEC of 31 March 1992 relating to tyres for motor vehicles and their trailers and to their fitting". THE COUNCIL OF THE EUROPEAN COMMUNITIES. Retrieved 2014-12-06.
  15. "How to calculate speed ratings". Auto Motor Öl. Retrieved 2014-12-06.
  16. "Bridgestone Product Reference Guide",The Bridgestone Tire & Rubber Co.
  17. 1 2 3 4 5 6 7 8 "Bridgestone Product Reference Guide",The Bridgestone Tire & Rubber Co.,Core Tire Knowledge: Glossary
  18. "FAQ". Cooper Tire.
  19. "Bridgestone Product Reference Guide",The Bridgestone Tire & Rubber Co.,Technical Bulletins: Tire Inspection Guidelines
  20. "Tire care -- Match mounting". Yokohama Tire Corporation.
  21. "2010 Year Book",The Tire and Rim Association, Inc.
  22. "Vintage Tire Size Conversion Chart".
  23. "Tire Size Conversion Chart".

External links

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