ISO Metric Sizing
Older Sizing Systems
A typical tire sold in the United States today might be marked as shown in the illustration. Click on any of the markings for an explanation of its meaning.
The commonest system is P metric system. This differs from the Metric system of markings. Further, P metric sizes are not the same as or interchangeable with LT metric sizes.
The type of tire a car requires will be found on a label attached either to the edge of the driver's door, or to the "B pillar", the support just behind the driver's door.
Any letters at the beginning:
|P||the tire is meant for a Passenger car.|
|LT||the tire is meant to be used on a Light Truck. Oops! Not a P metric tire.|
The first number, in this case “185,” is the nominal width of the tire in millimeters, from sidewall to sidewall, when it is properly mounted on a wheel and inflated but not supporting a car. top
The number after the slash, in this case “70,” is a percentage: the percentage the distance from the rim to the tread (in a properly mounted and inflated tire not supporting a car) is of the width of the tire. In this example it is 70, so 70% of 185 = 129.5; from the rim to the road is 129.5 mm (about 5.1 inches). Basically, the number tells you how tall the tire is; it is usually called the aspect ratio or the profile of the tire. top
The first letter after the numbers indicates the highest sustained speed at which the tire can be safely used, i.e., how fast you can safely go as far as the tires are concerned. Notice that the codes don't increase in alphabetic order.
|Code||indicates the tire must not be run at speeds over|
|in miles per hour||in kilometers per hour|
The “Z” or “ZR” rating identifies tires that may be run at speeds over 240 km per hour (149 mph); the “W”, "Y" and “(Y)” ratings are actually subcategories of “Z”.
Some tires lack a speed rating. A snow tire without a speed rating is actually a Q, 160 km per hour or 100 mph. A highway passenger car tire without a rating is limited to 170 km per hour (105 mph). A light truck (LT) tire is limited to 140 km per hour (87 mph).
The second letter describes the way the tire cords are laid:
|R||a radial, that is, the cords of the belting are at about 90 degrees to the tread.|
|D||a diagonal ply tire; the cords are about 45 degrees to the edge (the D stands for diagonal).|
The last number in the mark (in this case "13") is the diameter of the rim in inches.
The Uniform Tire Quality Grading System was developed by the U.S. Department of Transportation to give consumers some guidance on tire quality. At first the three ratings were printed together, like the "200AB" in the example at the top of the page, which was rather mysterious to the average consumer. Now the ratings are clearly labeled:
The actual ratings are determined by the manufacturer of the tire, not the Department. These ratings are most useful in comparing different models of one maker, rather than models from different makers.
In the United States, the only passenger car tires that need not show these ratings are snow tires, deep tread tires, temporary spares, tires with nominal rim diameters between 10 and 12 inches, and certain limited production tires.
49 CFR 575.104(c)(2)
The number “200” in the example is the treadwear index. The tread of a tire with a treadwear index of 200 should last twice as long as one with a treadwear index of 100. The life of the “reference tire,” the one with an index of 100 to which all others are compared, is determined by convoys of 4 cars driving for 6,400 miles over public roads near San Angelo, Texas. Because tire life depends on many factors other than the tire, such as the weight of the vehicle, how it is driven, tire pressure, and so on, there is no accurate way of determining from the treadwear index how many miles you'll get. However, under the highly controlled test conditions, a tire with a treadwear index of 100 lasts for about 30,000 miles. In practice, treadwear indexes below 200 are low, over 400 high. Some tires with indexes in the 500s are available, while the special tires for the rear wheels of the Acura NSX are rated 120 (the rubber compounds that give the best traction also wear the fastest).
For a detailed description of the procedure for determining the treadwear index, see www.ecfr.gov/cgi-bin/text-idx?SID=e25ce7767c24888019b4f7e765912faa&mc=true&node=se49.7.575_1104&rgn=div8http://www.ecfr.gov/cgi-bin/text-idx?SID=e25ce7767c24888019b4f7e765912faa&mc=true&node=se49.7.575_1104&rgn=div8"
The first letter after the treadwear index is a traction score that rates the tire's ability to stop on wet pavement. It is either AA, A, B or C, with AA the highest rating.
The second letter after the treadwear index is a temperature grade that rates the tire's resistance to the generation of heat and its ability to dissipate heat. It is either A, B, or C, with A the highest rating. The C rating is the minimum performance that passes Federal Motor Vehicle Safety Standard No. 109.
If the tire illustrated were a snow tire, it would bear the letters “M/S”, “M+S” or “M&S”, (for Mud and Snow), and a pictogram of a snowflake in the outline of a mountain. This code is promulgated by the Rubber Manufacturers Assn.
Similar codes are “AT”, for All Terrain, and “AS” for All Season. These codes have different meanings and the tires are not interchangeable.
(In this example, “B9PA B55X 101,” though usually prefixed with “DOT”, and usually called the “DOT code” or “TIN”). This code, assigned by the Department of Transportation, indicates who manufactured the tire, where, and when. There are too many manufacturers and places of manufacture codes for us to give them here; besides, they frequently change. Please see the most recent edition of:
Who Makes It and Where Directory
Boca Raton, FL: Tire Guides, Inc.
The date of manufacture is indicated by the last group of digits, which usually appear in a sunken rectangle. Before 2000, this group had three digits; since 2000, it has had four. The first two digits are the week of the year (01 = the first week of January). The third digit (for tires made before 2000) is the year (1 = 1991). For tires made after 2000, the third and fourth digits are the year (04 = 2004). The date of manufacture is essential information because tires deteriorate even if they are not used. European automobile manufacturers recommend that tires older than 6 years, including spares never driven upon, be discarded.
The maximum load and pressure ratings are also usually indicated on the sidewall, as well as information on the number of plies.
Some tires have an arrow molded into the sidewall to show the direction the tire must rotate when the car is moving forward. Such a tire has a tread pattern designed to be especially effective in wet weather. To achieve this, the tread is not symmetrical front to back, so, unlike most tires, it makes a difference which way the tire is rolling.
This system developed from the older Metric Sizing System, which in turn was a conversion of the Numeric Sizing System from inches to millimeters. A typical rating might be 185/60R14 82H, where
The big difference is the load index, which is required in Europe but not in the United States. The load index is the maximum weight each tire is expected to support at maximum speed.
The tire sizing system described above, known as P-Metric, came into use around 1976. Various systems were used before that, and tires are still manufactured to fit earlier designations.
|Name of system||A typical designation||Period of use|
|alpha-numeric||GR78-14||1968 to 1976|
|before about 1924|
In Europe similar designations were used, except that the tire's width was given in millimeters. An “R” was inserted to indicate a radial tire, for example, “185R15”.
Another style: 31 by 10.50R-15, sometimes written as 31-1050R15, is used for light truck tires.
In the early days of motoring, tire sizes were given as the nominal rim diameter in inches by the nominal rim width in inches. In the late 1920s, the order was inverted; but because the larger number is always the rim, the change caused little confusion.
Tire and Rim Association.
Rubber Manufacturers Association, www.rma.org
European Tyre and Rim Technical Organization (ETRTO)
Japan Tire Manufacturers Association (JATMA)
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Last revised: 12 March 2013.