Stewart & Stafford
Tire and Custom Wheel

Speed Ratings      Tire Size      DOT Symbols     Proper Inflation

Principle Tire Constructions

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Radial Tires

Body cords run across the tire perpendicularly to the beads. Radial tires have belt plies, which run nearly circumferentially around the tire, under the tread to stabilize the tread, strengthen the tread area and add flexibility to the sidewall. By restricting tread movement during contact with the road, the belt plies improve tread life.

Diagonal (Bias) Tires

Have two, four, or more body plies, which cross at an angle to the center line of the tread, to strengthen both the sidewall and the tread.

Belted Bias Tires

Have a body similar to that of bias tires, plus two or more belts under the tread to strengthen and stabilize the tread. The belts improve tread life by reducing tread movement during contact with the road.

Stewart & Stafford
Tire and Custom Wheel
(606) 437-4997

Air pressure - Forces exerted by air within tire, expressed in pounds per square inch.

Anchored Beads - Ply and flipper construction which locks beads in place in the tire. Rubber-coated cord fabric is wrapped around high tensile strength steel wire which has been insulated with rubber. The ends of the fabric, which are several inches in length are then carried far up into the sidewall of the carcass and locked in by the plies when the tire is vulcanized.

Aspect Ratio - A numerical term which expresses the relationship between the standing height of the tire and the cross section width. ( Aspect Ratio of 70 means the tire section stands approximately 70 percent as high as it is wide between the sidewall).

Bead - That part of the tire that is shaped to fit the rim. Made of high tensile steel wires that are wrapped in woven fabric and then held by the plies.

Carrying Capacity - Maximum rated tire load for a given inflation pressure as established by the Tire and Rim Association.

Chafer - Reinforcing fabric and rubber around the bead in the rim flange area to prevent chafing of the tire by rim parts.

Inner Liner - The layer of rubber with is laminated to the side of a tubeless tire to insure the air retention quality of the tire body.

Load Carrying Capacity - Maximum load for which tire is designed under standard conditions.

Load Range - A system of designations which identifies the carrying capacity range of a tire. These markings shown on the tire indicate the ply rating established for the tire. Passenger tire designations are B (4-ply rating), C (6-ply rating), and D (8-ply rating). This system was established by the Rubber Manufacturers Association and is used on all American-made Tires. Replaces ply rating.

O.D. (Outside Diameter) - Measurement of tire when inflated and mounted on rim.

Ply - Layer of rubber-coated parallel cords forming unit of tire carcass.

Polyester Cord - A synthetic fiber that excels in maintaining strength properties at high heat levels, and eliminates flat spotting.

Radial Ply - A tire with cords running radially from bead to bead (90 degrees to centerline of the tire).

Rayon - An artificial fiber made from cotton or wood pulp by chemical process.

Revolutions Per Mile (.) - The Number of Revolutions that the mounted tire will make in one mile, at rated load and inflation.

Rolling Resistance - The resistance of a tire to free rolling.

Section Width - Measurement of distance through cross sectional width of a tire at widest part, exclusive of scuffing rib when inflated to normal pressure and not under load.

Shoulder - Out edges of tread

Sipe - To cut across a tire tread to produce biting traction edges.

Speed Rating - A speed Designation of S, T, H, V, or Z shown in the size marking of some tires.

Stability - Ability of tires to maintain direction of vehicle on curve without causing excessive sway.

Tread Depth - The distance in thirty - seconds of an inch measured from the tread surface to the bottom of the grooves in a tire.

Tread Design - The pattern of the tread area.

Tread Elements - The parts of the tread design which are separated from each other and made distinct by the sipes and rib or lug design molded into the tire.

Tread Life - Length of service in miles before tread wears out.

Tread Wear Indicators - Narrow bars of rubber molded at a height of 2/32" across the bottom of the tread grooves. When the tread wears down to these bars, the tire should be replaced.

Underinflation - Tire having less than recommended air pressure for the load being carried.

Weather Checking - Fine hairline cracks in surface of rubber, caused by oxidation and other atmospheric effects.

Stewart & Stafford
Tire and Custom Wheel
(606) 437-4997

Size designation systems presently in use are: P-Metric, European Metric, LT-Metric, Alpha-Numeric, Numeric, Flotation, and T Type Temporary Spare.

Size designations usually include letters, as well as numbers, which have the following meanings:

R--Radial Construction
 B--Belted Bias Construction
 D--Diagonal (Bias) Construction
 P--Passenger Car
 T--Temporary Spare
 LT--Light Truck

(note: Other letters denoting speed symbols, "S", "T", "H", "V" and "Z" may appear in the size designation)

Tire Sidewall Markings

• Tire Size Designation - The speed rating symbol can be included as part of the tire size designation. For example the tire size P205/60HR15 has the following meaning:
• P - Passenger car tire
   205 - Nominal section width in millimeters
   60 - Nominal aspect ratio (section height-to-width ratio)
   H - H speed rated tire ( 130 mph max. speed rating)
   R - Radial tire construction
   15 - Nominal rim diameter in inches
• Service Description - This is a new marking that is beginning to appear on speed rated tires, which is intended to become a worldwide standard eventually. It is distinct and separate from the tire size designation. It consists of a two- or three-digit load index and a speed symbol.

 A typical service description is:
 Load Index - 90 H - Speed Symbol

•  Load Index - A numerical code (90 in this example) associated with the maximum load a tire can carry at the speed indicated by its Speed Symbol under specified service conditions. In the United States the maximum tire load capacity is stamped in pounds ( and in may cases, also in kilograms) in the lower sidewall of the tire.
• Speed Symbol - In this example, the speed symbol H in the service description means a maximum speed rating of 130 mph. If a tire were S, T or V speed rated, an "S", "T" or "V" could appear in the service description in the same position as the "H" in the above example.
• Markings - (Examples - 1. P205/60HR15, 2. P205/60HR15 90H, 3. P205/60R15) For a transition period ending December 31, 1990, speed rated tires may be marked like any of the examples. However, the worldwide tire industry is committed to standardizing the tire size and service description markings to be in accord with international standards. Therefore, at the end of the transition period, any speed symbol denoting a fixed minimum speed rating will appear in the service description as shown in example 3.

Selection of Speed Rated Replacement Tires

• Selection - When replacement of tires is desired, consult the vehicle placard (normally located on door frame or glove box door) or the owner's manual for correct size. If the tires shown on the vehicle placard do not have speed ratings, speed rated tires may be applied, if desired. When the placard tire size nomenclature contains a speed symbol (For example, P205/60HR15) the replacement tire must have the same or higher speed rating if the speed capability of the vehicle is to be maintained. (If the replacement tire is not speed rated, the speed capability of the vehicle may be limited by the speed capability of the replacement tire. Consult tire manufacturer.) Follow the recommendations contained in RMA publications for other considerations in the selection of replacement tires.
• Mixing - be sure to follow RMA rules on tire mixing. If tires of different speed ratings are mounted on a vehicle, the tires with the lower speed rating will limit permissible tire related vehicle speed. If any one of the replacement tires is to speed rated, the speed capability of the vehicle may be limited by the speed capability of that tire. Consult tire manufacturer.
• Retread & Repair of Speed Rated Tires - Consult tire manufacturer regarding the alteration (such as repair of retreading) of Speed Rated Tires.

Stewart & Stafford
Tire and Custom Wheel
(606) 437-4997

The federal Government Uniform Tire Quality Grading Standard applies to passenger tires only (but excludes deep tread, winter type snow tires, temporary use spare tires, and tires with nominal rim diameters of twelve inches or less). Tires subject to the standard are required to be graded on the performance factors of treadwear, traction, and temperature. The grades are molded on the tire sidewall, and, in addition for replacement tires, a label affixed to the tread lists and explains these grades.

All passenger car tires must conform to Federal Safety Requirements in addition to these grades.

• Treadwear - The Treadwear grade is a comparative rating based on the wear rate of the tire when tested under controlled conditions on a specified government test course. For example a tire graded 150 would wear one and a half times as well on the government course as a tire graded 100. The relative performance of tires depends upon the actual conditions of their use, however, and may depart significantly form the norm due to variations in driving habits, service practices and differences in road characteristics and climate.
• Traction - The traction grades, from highest to lowest are A, B, and C, and they represent the tire's ability to stop on wet pavement as measured under controlled conditions on specified government test surfaces of asphalt and concrete. A tire marked C may have poor traction performance. WARNING: The traction grade assigned to this tire is based on braking (straight ahead) traction test and does not include cornering (turning traction).
• Temperature - the temperature grades are A (highest), B, and C, representing the tire's resistance to the generation of heat and it's ability to dissipate heat when tested under controlled conditions on a specified indoor laboratory test wheel. Sustained High temperatures can cause the material of the tire to degenerate and reduce tire life, and excessive temperatures can lead to sudden tire failure. WARNING: The temperature grade for this tire is established for a tire that is properly inflated and not overloaded. Excessive speed, underinflation, or excessive loading, either separately of in combination, can cause heat buildup and possible tire failure.

Speed Ratings for Passenger Car Tires

Speed ratings of tires are identified by means of a speed symbol. The meanings of these symbols are explained below.

Although a tire may be speed rated, we do not endorse the operation of any vehicle in an unsafe or unlawful manner. Speed ratings are based on laboratory tests which relate to performance on the road, but are not applicable if tires are underinflated, overloaded, worn out, damaged or altered. Furthermore, tire speed ratings do not imply that vehicles can be safely driven at the maximum speed for which the tire is rated, particularly under adverse road and weather conditions or if the vehicle has unusual characteristics.

Stewart & Stafford
Tire and Custom Wheel
(606) 437-4997

Tires are designed to give many thousands of miles of excellent service. But you must take good care of tires in order to get the maximum benefit from them. The most important factors in caring for tires are : Inflation Pressure, Loading, Driving with care.

Highway vehicles have tire/loading information placards affixed to the door edge, door post or on the glove box door that contain the following important information: 1. Tire Size; 2. Cold tire inflation pressures; 3. Gross axle weight ratings (GAWR) for each axle.

Proper tire inflation is shown on the vehicle's tire placard. Once a correct tire size is mounted on the vehicle, proper inflation is the most important requirement for tire mileage, safety, and performance. Correct tire inflation provides proper sidewall deflection and safe operating temperatures for the tires. Underinflation creates excessive heat, lowers load carrying capacity and can seriously reduce tire life and cause tire failure. In addition, underinflation increases rolling resistance of tires and, therefore, increases fuel consumption.

It is impossible to determine whether tires are properly inflated just by looking at them. If a tire is underinflated it will run hotter, wear faster and or unevenly, adversely affect vehicle handling, and may fail in service. Always check your presure with a good quality air guage or stop by and let us do it.