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Tire Facts
 
Tire Facts

 

Tire Facts

What you should know about those labels on your new tires.

Exactly what do those codes, temperature ratings, traction, and tread depth mean? What is it all about? This guide makes it simple for the un-educated tire consumer.

Remember tire safety starts with "You pump' em, do not thump' em", "Use an air gauge", and "Check your air pressure regularly". Also visually inspect the tread and sidewall of the tire for any abnormalities. Keeping your tires properly inflated and maintained will save you gas and more. Check your door jam for manufacturer's specifications as well as your owner's manual and the tire side wall if not OEM for inflation instructions.

Tire Size Markings

The tire size shown below is P185/60R14 82H. The P represents the car type, Passenger. The 185 represents its section width (tire width in mm). The 60 is the tires Aspect Ratio (the ratio of the sidewall height to the tread width). The R represents radial tire construction. The 14 represents the rim/wheel size and 82H represents the load index and speed symbol.

Speed Ratings

Speed ratings are determined by indoor laboratory testing methods which measure high speed tire durability under controlled test conditions. These test procedures do not take into account underinflation, tire damage, vehicle characteristics, or road conditions which can lead to sudden tire failure or loss of vehicle control at much lower speeds than indicated by the tire's speed rating. The validity of using speed rated tires in the U.S. is based on the idea that the tire's top speed capability must at least equal the vehicle's top speed capability, since it cannot be assumed that the driver will always observe the speed limit.

Recently the speed rating is being referred to as the "performance rating" of the tire, since the higher speed rated tires generally offer improved handling and maneuverability compared to lower speed rated tires.

Load Index

Load index indicates the maximum load capacity each tire is designed to support. Like speed ratings, assume near perfect operating conditions to obtain the ratings listed in the table below.

Uniform Tire Quality Grading (UTQG)

Under UTQG, tires are graded by the manufacturers in three areas; treadwear, traction and temperature resistance.

  1. 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 1.5 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, and may depart significantly from the norm due to variations in driving habits, vehicle condition, road characteristics, and climate.
     
  2. Traction 

    The traction grades from highest to lowest are AA, A, B and C. They represent a 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.
     
  3. Temperature

    The temperature grades are also A, B and C, representing the tire's resistance to heat generation and its ability to dissipate heat when tested under controlled conditions on a specified indoor laboratory test wheel. Sustained high temperature can cause the material of the tire to degenerate and reduce tire life; excessive temperature can lead to sudden tire failure. The grade C corresponds to a level of performance which all passenger tires must meet under Federal safety laws.

Wheel Basics

  1. Wheel Width

    Distance between inside of flanges rounded to nearest 1/2-inch. Ensure that wheel width is proper for tire size you intend to mount on it. All tire sizes have minimum and maximum wheel width limits. Correct wheel width is about 75% of tire cross section width.

     
  2. Wheel Diameter

    Distance from bead seat to bead seat across diameter of wheel. Must be exactly the same as tire rim diameter. Mounting a tire of one diameter on a wheel of another diameter can result in violent explosion causing serious injury or even death. Always verify diameter stamped on the wheel and match the tire exactly.

     
  3. Wheel Offset

    Distance between wheel mounting surface where bolted to hub of drum and centerline of rim. Determines vehicle "track" or distance between tires on each axle. Wheels with more negative offset than original wheels move outboard on car. Keep the wheel offsets as close to original as possible to avoid steering difficulties or wheel bearing fatigues. Negative offset on rear increases "track" and may improve stability and handling.
     

Ply Rating/Load Range

While there is no industry-wide definition of ply rating, truck tires are frequently marked with ply rating and equivalent Load Range. These markings are used to identify the load and inflation limits of that particular tire, when used in a specific type of service.

See the Department of Transportations national highway traffic safety administrations, vehicle in use standard for tires below.

Key To Tire Data


 Volume 6]
[Revised as of October 1, 2005]
From the U.S. Government Printing Office via GPO Access
[CITE: 49CFR570.9]

[Page 199]

                        TITLE 49--TRANSPORTATION

                   CHAPTER V--NATIONAL HIGHWAY TRAFFIC
                    SAFETY ADMINISTRATION, DEPARTMENT
                            OF TRANSPORTATION

PART 570_VEHICLE IN USE INSPECTION STANDARDS--Table of Contents

          Subpart A_Vehicles With GVWR of 10,000 Pounds or Less

Sec. 570.9  Tires.

    (a) Tread depth. The tread on each tire shall be not less than two
thirty-seconds of an inch deep.
    (1) Inspection procedure. Passenger car tires have tread depth
indicators that become exposed when tread depth is less than two thirty-
seconds of an inch. Inspect for indicators in any two adjacent major
grooves at three locations spaced approximately equally around the
outside of the tire. For vehicles other than passenger cars, it may be
necessary to measure tread depth with a tread gauge.
    (b) Type. Vehicle shall be equipped with tires on the same axle that
are matched in tire size designation, construction, and profile.
    (1) Inspection procedures. Examine visually. A major mismatch in
tire size designation, construction, and profile between tires on the
same axle, or a major deviation from the size as recommended by the
manufacturer (e.g., as indicated on the glove box placard on 1968 and
later passenger cars) are causes for rejection.
    (c) General condition. Tires shall be free from chunking, bumps,
knots, or bulges evidencing cord, ply, or tread separation from the
casing or other adjacent materials.
    (1) Inspection procedure. Examine visually for conditions indicated.
    (d) Damage. Tire cords or belting materials shall not be exposed,
either to the naked eye or when cuts or abrasions on the tire are
probed.
    (1) Inspection procedures. Examine visually for conditions
indicated, using a blunt instrument if necessary to probe cuts or
abrasions.

[38 FR 23950, Sept. 5, 1973, as amended at 39 FR 12868, Apr. 9, 1974; 39

FR 19781, June 4, 1974]

 

  
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