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.
- 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.
- 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.
- 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
- 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.
- 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.
- 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. The table shows the conversion of tire markings.
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