Tire Topics – Tires 101
Tire Dimensions
- Static Loaded Radius: distance from the center of the axle to the ground under the specified load and inflation pressure.
- Rim Diameter: diameter of the rim from bead seat to bead seat.
- Overall Diameter: diameter of the tire from tread surface to tread surface while inflated but unloaded.
- Overall Section Width: distance between the outer sidewalls of an inflated tire.
- Rim Width: distance between the inside of the rim flanges.
- Section Height: distance from the bead seat to the outer tread surface of the inflated tire.
- Section Width: distance between the outer sidewalls of an inflated tire, less any ornamentation or curb ribs.
- Tread Width: the width of the tread surface, designed for contact with the road.
Speed Ratings
Many of today’s automobiles come equipped with speed rated Original Equipment tires. Speed rating passenger tires originated in Europe, where highway speeds can exceed 100 mph (160kph). The speed rating indicates the maximum speed a properly inflated tire will withstand for a determined time. Testing for speed rating certification is conducted in a laboratory setting.
NOTE: The speed rating is void if the tires are worn out, damaged, repaired, retreaded, or otherwise altered from their original condition. If tires are repaired, retreaded, or otherwise altered, they should not be operated at higher than normal highway speeds.
| SPEED SYMBOL | SPEED MPH | SPEED KPH |
|---|---|---|
| M | 81 | 130 |
| N | 87 | 140 |
| P | 93 | 150 |
| Q | 99 | 160 |
| R | 106 | 170 |
| S | 112 | 180 |
| T | 118 | 190 |
| U | 124 | 200 |
| H | 130 | 210 |
| V*(VR) | 149 | 240 |
| W**(ZR) | 168 | 270 |
| Y**(ZR) | 186 | 300 |
*Current tire speed rating markings include the use of the service description to identify the tire’s speed capability (P215/65R15 95V – maximum speed 149 mph).
**Any tire with a speed capability above 149 mph (240 kph) can, at the tire manufacturer’s option, include a “ZR” in the size designation (P275/40ZR17). If a service description IS NOT included with the size description, the tire manufacturer must be consulted for the maximum speed capability (P275/40ZR17–speed capability is >149mph). If a service description IS included with the size description, the speed capability is limited by the speed symbol in the service description (P275/40ZR17 93W = maximum speed 168 mph (270kph).
Speed Rating Characteristics
Tires have always had the ability to change an automobile’s driving characteristics. Technology has given the tire an important role as a component of the automobile suspension. The speed rating of the tire is an indicator of the tire’s performance capability. Improving the tire’s performance capability will normally benefit an automobile’s performance. Conversely, decreasing the tire’s speed rating generally lowers the tire’s ability to contribute to the automobile’s performance. Changing from the O.E. tire speed rating to another performance level tire will probably change handling in areas such as steering response, braking, traction, cornering, and evasion/recovery.
Uniform Tire Quality Grading (UTQG)
Quality grading is designed to make the tire purchase decision easier for you. Ideally, the system is intended to provide simple, comparative data which you can use in making an intelligent buying decision. However, the ratings are based upon test results achieved under very special conditions. As a result, misinterpreting the comparative data as it relates to your particular driving habits, conditions, etc., is a possibility. You should still rely upon your service or tire professional for assistance.
Quality grading designates the comparative performance levels of a tire based upon government-specified tests, but commissioned by the individual tire manufacturers. All tire manufacturers and brand name owners are required to grade regular and all season passenger tires in three categories: Treadwear, Traction and Temperature.
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 to 6,000 miles (9600 km). 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 from the norm due to variations in driving habits, service practices and differences in road characteristics.
Traction AA, A, B, C
The traction grades from highest to lowest, are AA (the highest) 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 will have the lowest traction performance.
WARNING: THE TRACTION GRADE ASSIGNED IS BASED ON A WET BRAKING (STRAIGHT AHEAD) TRACTION TEST AND DOES NOT INCLUDE CORNERING (TURNING) TRACTION.
Temperature A, B, C
The temperature grades are A, B, and C, representing the tire’s resistance to the generation of heat 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, and excessive temperature can lead to sudden tire failure. The grade C corresponds to a level of performance that all passenger car tires must meet under the Federal Motor Vehicle Safety Standard No. 109. Grades A and B represent higher levels of performance on the laboratory test wheel than the minimum required by law.
WARNING: THE TEMPERATURE GRADE IS ESTABLISHED FOR A TIRE THAT IS PROPERLY INFLATED AND NOT OVERLOADED. EXCESSIVE SPEED, UNDER INFLATION, OR EXCESSIVE LOADING, EITHER SEPARATELY OR IN COMBINATION, CAN CAUSE HEAT BUILDUP AND POSSIBLE TIRE FAILURE.