Alphanumeric code specifying tire sizes and limits
Assorted new car tires with a variety of tread patterns and different sizes
Automotive tires are described by an alphanumeric tire code (in North American English) or tyre code (in Commonwealth English), 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 SI and USC units, and ad-hoc extensions to lettering and numbering schemes. New automotive tires frequently have ratings for traction, treadwear, and temperature resistance, all collectively known as the Uniform Tire Quality Grading.
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.
The DOT code[1] is an alphanumeric character sequence molded into the sidewall of the tire and allows the identification of the tire and its age. The code is mandated by the U.S. Department of Transportation[2] but is used worldwide.[3] The DOT code is also useful in identifying tires subject to product recall[4] or at end of life due to age.
Since February 2021 UK vehicle Regulations do not permit the use of tyres over 10 years old on the front steered axle(s) of heavy goods vehicles, buses and coaches.[5] The ban also applies to all tyres in single configuration on minibuses. In addition, it is a requirement for all tyres on these vehicles to display a legible date code.
The European Tyre and Rim Technical Organisation (ETRTO) and the Tire and Rim Association (TRA) are two organizations that influence national tire standards. The objectives of the ETRTO include aligning national tire and rim standards in Europe.[6] 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.[7] 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).[8] Canada has published tire regulations, such as the Motor Vehicle Tire Safety Regulations SOR 95-148.[9]
All tires sold for road use in Europe after July 1997 must carry an E-mark. The mark itself is either an upper case "E" or lower case "e" – followed by a number in a circle or rectangle, followed by a further number. An (upper case) "E" indicates that the tire is certified to comply with the dimensional, performance and marking requirements of ECE Regulation 30. A (lower case) "e" indicates that the tire is certified to comply with the dimensional, performance and marking requirements of Directive 92/23/EEC. The number in the circle or rectangle denotes the country code of the government that granted the type approval. The last number outside the circle or rectangle is the number of the type approval certificate issued for that particular tire size and type.[10]
Tire identification diagram Tire diameter (inches) = (xxx*yy/1270)+zz, where tire size is commonly written "Pxxx/yyRzz" on the sidewall. Note, "xxx" is tread width in mm. "yy" is "aspect ratio" (sidewall height to tread width). "zz" is rim diameter (inches).
The ISO metric tire code consists of a string of letters and numbers, as follows:[11][12]
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)
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.[13]
3-digit number: 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 character for character separation.
2- or 3-digit number: 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% (if written, it should be like xxx/82). If the number is larger than 200, then this is the diameter of the entire tire in millimeters.
An optional letter indicating the speed rating of the tire. Alternatively, the letter may appear at the end, following the load index. If the letter here is Z, indicating a maximum speed in excess of 240 km/h (149 mph), then a more specific letter W or Y may appear after the load index (see speed rating, below).
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)
1- or 2-digit number: 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.[14][15]
2- or 3-digit number: Load index; see table below. Some light-truck tires are approved for "dual use", that is they can be run in pairs next to each other. If so, separate load indexes will be specified for single and dual usage. In the example shown in the light-truck tire illustration, the tire has a load index of 114 if used as a single tire, and a load index of 111 if used in a pair.[16] Tires without this designation are unsafe for dual usage.
1- or 2-digit/letter combo: Speed rating; see table below
Tire identification diagram, light truck specific features
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 of 92%, while section widths not ending in zero (e.g., 7.05 or 10.55) indicate the aspect ratio of 82%. These aspect ratios often vary from today's tire manufacturer specification.
Construction of the fabric of the tire:
B: bias belt
D: diagonal
R: radial
2-digit number: Diameter in inches of the rim that this tire is designed to fit.
LT: Designates that this is a light truck tire.
Load index and speed rating are sometimes not mandatory for flotation sizes, but must be for any tire approved for street and highway use.
2- or 3-digit number: Load index; see table below.
1- or 2-digit/letter combination: Speed rating; see table below.
Additional marks: see subheading below.
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 tire diameter 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.[17]
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.[19]
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).[20]
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).[12] 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.[20]
Standard load table (extract from ETRTO standards manual – 2010 page G7 ref 42 psi (290 kPa))[12]
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
1,700 kg (3,700 lb)
67
307 kg (677 lb)
87
545 kg (1,202 lb)
107
975 kg (2,150 lb)
127
1,750 kg (3,860 lb)
68
315 kg (694 lb)
88
560 kg (1,230 lb)
108
1,000 kg (2,200 lb)
128
1,800 kg (4,000 lb)
69
325 kg (717 lb)
89
580 kg (1,280 lb)
109
1,030 kg (2,270 lb)
129
1,850 kg (4,080 lb)
70
335 kg (739 lb)
90
600 kg (1,300 lb)
110
1,060 kg (2,340 lb)
130
1,900 kg (4,200 lb)
71
345 kg (761 lb)
91
615 kg (1,356 lb)
111
1,090 kg (2,400 lb)
131
1,950 kg (4,300 lb)
72
355 kg (783 lb)
92
630 kg (1,390 lb)
112
1,120 kg (2,470 lb)
132
2,000 kg (4,400 lb)
73
365 kg (805 lb)
93
650 kg (1,430 lb)
113
1,150 kg (2,540 lb)
133
2,065 kg (4,553 lb)
74
375 kg (827 lb)
94
670 kg (1,480 lb)
114
1,180 kg (2,600 lb)
134
2,125 kg (4,685 lb)
75
387 kg (853 lb)
95
690 kg (1,520 lb)
115
1,215 kg (2,679 lb)
135
2,185 kg (4,817 lb)
76
400 kg (880 lb)
96
710 kg (1,570 lb)
116
1,250 kg (2,760 lb)
136
2,245 kg (4,949 lb)
77
412 kg (908 lb)
97
730 kg (1,610 lb)
117
1,285 kg (2,833 lb)
137
2,305 kg (5,082 lb)
78
425 kg (937 lb)
98
750 kg (1,650 lb)
118
1,320 kg (2,910 lb)
138
2,365 kg (5,214 lb)
79
437 kg (963 lb)
99
775 kg (1,709 lb)
119
1,360 kg (3,000 lb)
139
2,435 kg (5,368 lb)
Some of the older letter-code load-range ratings for Light Truck Tires can be found in a chart[21] published by the Goodyear Tire & Rubber Company. For example:
The speed symbol is made up of a single letter or an A with one numeral. It indicates the maximum speed at which the tire can carry a load corresponding to its load index.[12] The testing method consists of pressing the tire against a large diameter metal drum to reflect its appropriate load, and run at ever increasing speeds in 10 km/h (6.2 mph) steps in 10 minute increments until the tire's required speed has been met.[12]
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).
Tires with a speed rating higher than 300 km/h (186 mph) 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.[22] In this case it is possible to calculate the appropriate speed rating with a formula.[23]
Tires with a higher speed rating generally have a better grip but a lower tread life, reduced performance in cold weather and reduced driving comfort.[24][25][26]
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.[27] 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.
Arrows: Some tread designs are "directional", and designed to perform better when driven in a specific direction. Such tires will have an arrow showing which way the tire should rotate when the vehicle is moving forwards.
B: Bias belted; tires for motorcycles (Example: 150/70 B 17 69 H) – diagonal construction with belt added under the tread
BSB: Broken serrated band
BSL: Black serrated letters
BSW: Black sidewall
C: Commercial; tires for light trucks (Example: 185 R14 C)
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 (two digits for week of the year plus two digits for year; or two digits for week of the year plus one digit for year for tires made prior to 2000). Although not law, some tire manufacturers do not Tire manufactured in 10th week of 2001suggest using a "new" tire that has been sitting on the shelf for more than six years (Ford Motor Company) or 10 years (Cooper Tire citing a tire association recommendation).[29] JATMA, the Japanese Automotive Tyre Manufacturers Association recommends that all tires be inspected at five years, and all tires that were manufactured more than ten years previous be replaced.[30]
E4: Tire approved according to the ECE-regulations, the number indicating the country of approval.
"ELT": Pirelli Elect tyres, specific for electric car
"J": Jaguar original manufacturer fitment
LL: Light load; tires for light usage and loads
"M/C": Only for motorcycle fitment
M+S, or M&S: Mud and snow; A tire that meets the Rubber Manufacturers Association (RMA) and Rubber Association of Canada (RAC) all-season tire definition.[28] These are commonly found on all-season tires, with self-cleaning tread and average traction in muddy or very snowy conditions, and for low temperatures. Spike tires have an additional letter, "E" (M+SE).
A/T or AT: All Terrain; Designed for all conditions on and off road, master of none
M+T or M&T: Mud and terrain; Designed to perform in mud or on other terrain that requires additional traction such as on rocks, in deeper snow, and in loose gravel.
M/T or MT: Mud Terrain; Designed for deep mud and rock crawl
Made in ...: Country of production
MGT: Maserati Genuine Tire. Original tires for Maserati
Mountain snowflake pictograph: Winter passenger and light truck tires that meet the severe snow service requirements of Rubber Manufacturers Association (RMA) and Rubber Association of Canada (RAC).[28]
N-x: Original tires for Porsche where "x" is a "0" for the first approved in that size, "1" the second, ...[28]
"NHS": Not highway service
ORWL: Outlined raised white lettering
OWL: Outlined white lettering
RF: Reinforced – for Euro-metric tires, the term 'reinforced' means the same thing as 'extra load'[28]
RFT: Run-flat tire; Tires designed for vehicles without spare tires. Reinforced sidewalls allow the tire to be driven "flat" for a distance specified by the manufacturer (usually 50 miles)
RSC (inside a circle): BMW runflat system component[28]
RWL: Raised white lettering Examples of different tire sidewall markings
TT: Tube-type, tire must be used with an inner-tube
TWI: Tread wear indicator – a device, such as a triangle or a small Michelin Man icon, located where the tread meets the sidewall, and indicating the location of the raised wear bars in the tire tread channels – TWI is also used to refer to the raised wear bars themselves.
VSB: Vertical serrated band
WSW: White sidewall
XL: extra load; a tire that allows a higher inflation pressure than a standard load tire, which increases the tire's maximum load
ZP: zero-pressure; Michelin's branding for their run-flat models.
To facilitate proper balancing, most tire manufacturers also mark red circles (uniformity) and/or yellow dots (weight) on the sidewalls of their tires to enable the best possible match-mounting of the tire/wheel assembly.[32]
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) rims. 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 US 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 rim 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.[33] Note that the tire's cross-section and diameter are always specified when measured on a rim of a specified width; different widths will yield different tire dimensions.
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.[34]
From 1965 to the early 1970s, 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 rim had a separate sequence of load ratings; thus, a C78-14 and a C78-15 are not the same width. An aspect ratio of 78% was typical for letter-sized tires, although 70% was also common and lower profiles down to 50% were occasionally seen.[35]
