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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