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Reading Your Sidewalls From "Car Care & Repair" episode CCR-116

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The array of numbers on the sidewalls of your tire may at first appear as indecipherable as ancient hieroglyphics. In fact, they are fairly easy to interpret and provide a wealth of useful information. Tire retailers generally have pamphlets that will explain what the codes mean in fairly good detail. Figure A Modern tires feature a number of advanced characteristics such as steel-belted construction and sophisticated tread design. All of the terms, numbers and codes found in tire ads may seem baffling at first, but there is a method to the madness. Since you are certain to eventually be faced with a tire-purchasing decision, arm yourself with a little knowledge before visiting your tire retailer. Figure C Figure D Figure E The array of numbers on the sidewalls of your tire may at first appear as indecipherable as ancient hieroglyphics. In fact, they are fairly easy to interpret and provide a wealth of useful information. Tire retailers generally have pamphlets that will explain what the codes mean in fairly good detail.

When replacing worn-out tires, many people simply rely on the recommendation of the salesperson at the tire retailer when making their purchase decision. If you want to make a more informed choice, it's helpful to know something about the alphanumeric codes that are printed on the sidewalls of all commercial tires. This series of numbers and letters may seem baffling at first, but it's actually fairly simple to decode, and each element conveys some useful information. If you don't want to commit all of the codes to memory, simply ask your tire retailer for a brochure or leaflet that explains them. Retailers are required by law to make this information available.

Note: A diagram explaining tire codes may be viewed with

Tire codes

Looking at the sidewall of a tire, you'll see a series of raised numbers and letters such as P 205/55 R 16 88V. Following is a summary of the information you can gain from those markings.

• Type of vehicle. The letter at the beginning of the series (figure A) designates the type of vehicle for which the tire is intended to be used. The P in our example indicates that the tire is intended for a passenger vehicle. Other designations include "LT" for light truck and "T" for a temporary or spare tire.

• Tire width. The first number in the series denotes the width of the tire, measured across the tread from sidewall to sidewall (figure B). The measurement is expressed in millimeters. In our example, the tire is 205 millimeters wide. The appropriate tire width relates to the width of the rim on which it will be mounted.

• Aspect ratio. Following the slash mark, the second number in the series (figure C) denotes the ratio of the tire's sidewall measurement to its height. In our example the 55 means that this tire's height is 55 percent of its width. Tires with lower numbers are referred to as low-profile tires.

• Tire type or construction. The second letter in the series is known as the construction code and indicates the type of tire. More specifically, it indicates the type of ply construction in the tire's casing. In our example R indicates that this is a radial tire. Most tires today are steel-belted radial tires, meaning that the cords that make up the belt are made from steel. It also means that the cords making up the belt run radially around the tire -- at an angle perpendicular to the tire's bead. (The bead is the steel hoop made into the inner perimeter of the tire that holds the tire securely on the wheel-rim.) Other possible designations include "B" for belted-bias tires, "D" for diagonal or bias-ply tires or "E" for elliptical. These latter designations are far less common in contemporary tires.

• Wheel diameter. The next number is the diameter of the wheel's rim, measured in inches. In our example 16 means that the tire is made to fit on a wheel that is 16 inches wide.

• Load index. Following the wheel diameter is a number that corresponds to a maximum load capacity (figure D) -- the amount of weight the tire can safely carry assuming that each tire is filled to maximum inflation capacity. Finding the weight associated with each number requires looking up the number in a load-index chart. In our example, a tire with a load index of 88 has a maximum load-carrying capacity of 1,235 pounds. Most passenger-vehicle tires fall within a load-index range between 75 and 105. That index-range corresponds to load capacities ranging from 853 to 2,039 pounds. The load index is often given in association with a speed rating.

• Speed rating. Following the load index, some tires list a speed rating (figure E). This designation generally appears only on performance tires and indicates the maximum safe speed for which the tire may be used. The speed rating uses an alphabetic code with specific speeds associated with individual letters. Higher speeds are generally associated with letters toward the end of the alphabet. In our example, V indicates that the tire is certified for speeds up to 149 mph. Contrary to the assumption made by many consumers, the speed rating does not pertain to the tire's performance capabilities at high speeds. Rather, it is solely a measure of the tire's ability to withstand the heat generated by driving at high speeds. Tires with lower speed ratings will deteriorate faster as a result of heat if the vehicle on which they are used is frequently driven at high speeds. Not all tires are speed-rated, but if your car originally came with speed-rated tires, replacements should also be speed-rated. Following is a list of letters used in speed ratings and the speed in mph associated with each.

  F = 50 mph
  G = 56 mph
  J = 62 mph
  K = 68 mph
  L = 75 mph
  M = 81 mph
  N = 87 mph
  P = 93 mph
  Q = 100 mph
  R = 106 mph
  S = 112 mph
  T = 118 mph
  U = 124 mph
  H = 130 mph
  V = 149 mph
  Z = 149+ mph

• The tire code may contain one final element that is simply the manufacturer's name or a descriptive designation for the tire.

Additional information

In addition to the large, raised figures that make up the tire code, there is additional relevant information that you may find printed on the tire's sidewall. These remaining indicators will probably be embossed on the tire in smaller-size print along its inner or outer perimeter (figure F).

• The temperature rating is an indicator of how well the tire withstands heat buildup. "A" is the highest rating; "C" is the lowest.

• The traction rating is an indicator of how well the tire is capable of stopping on wet pavement. "A" is the highest rating; "C" is the lowest.

• The tread-wear rating gives a comparative rating for the useful life of the tire's tread. A tire with a tread-wear rating of 200, for example, could be expected to last twice as long as one with a rating of 100. Tread-wear grades typically range between 60 and 600 in 20-point increments. It is important to consider that this is a relative indicator, and the actual life of a tire's tread will be affected by quality of road surfaces, type of driving, correct tire inflation, proper wheel alignment and other variable factors.

• The DOT code indicates that the tire complies with safety standards set out by the U.S. Department of Transportation and is approved for highway use. The DOT code contains specific information about where and when an individual tire was made.

RESOURCES :

Auto Repair for Dummies
Author: Deanna Sclar
ISBN: 0764550896

Tires, Suspension and Handling
ISBN: 1560918314
Author: John C. Dixon
(Society of Automotive Engineers