Tires contain some very useful information molded into the sidewall. The following diagram shows what information's available and how to read it:
Shown here is the sidewall of a popular "P-metric", speed-rated auto tire. "P" stands for passenger, "215" represents the width of the tire in millimeters; "65" is the ratio of height to width; "H" is the speed rating; "R" means radial, and "15" is the diameter of the wheel in inches. Some speed-rated tires carry a Service Description, instead of showing the speed symbol in the size designation. The Service Description, 89H in this example, consists of the load index (89) and speed symbol (H).
A "B" in place of the "R" means the tire is belted bias construction. A "D" in place of the "R" means diagonal bias construction.
The maximum load is shown in lbs. (pounds) and in kg (kilograms), and maximum pressure in PSI (pounds per square inch) and in kPa (kilopascals). Kilograms and kilopascals are metric units of measurement. The letters "DOT" certify compliance with all applicable safety standards established by the Department of Transportation (DOT). Adjacent to this is a tire identification or serial number. This serial number is a code with up to 11 digits that are a combination of numbers and letters.
The sidewall also shows the type of cord and number of plies in the sidewall and under the tread.
The DOT requires the manufacturers to grade passenger car tires based on three performance factors: tread wear, traction, and temperature resistance.
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 test track as a tire graded "100". (Note: "Baseline" simply means the manufacturer's mid-level standards; some have higher or lower standards than others.)
- Less Than 100 - Poorer - 100 - Baseline - More Than 100 - Better
It is wrong to link tread wear grades with your projected tire mileage. The relative performance of tires depends upon the actual conditions of their use and may vary due to driving habits, service practices, differences in road characteristics and climate. The Treadwear grade simply lets you compare the potential life of various tires.
Treadwear grades are valid only for comparisons within a brand product line. They are not valid for comparisons across brands.
Traction grades 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 - Best - B - Intermediate - C - Acceptable
The Traction grade is based upon "straight ahead" braking tests; it does not indicate cornering ability.
Hard-rubber compounds give longer Treadwear, but poorer Traction. The Traction grade lets you compare Treadwear against Traction under wet weather conditions.
The temperature grades represent the tire's resistance to the generation of heat when tested under controlled conditions on a specified indoor laboratory test wheel.
- A - Best - B - Intermediate - C - Acceptable
The 3 Temperature grades represent the tire's
resistence to the generation of heat
ability to dissipate heat.
Sustained high temperatures can cause the materials of the tire to degenerate and thus reduce tire life. Excessive temperatures can also lead to sudden tire failure.
Federal law requires that all tires meet at least the minimal requirements of Grade C.
The speed rating indicator on the tire relates to the maximum sevice speed of the tire. Many radials are available in speed-rated versions to match the speed capabilities of the world's fastest cars. Generally, it is recommended that a speed-rated tire be replaced with a tire having an equivalent or greater speed rating.
However, if you never drive at speeds near the limits of your speed-rated tires, you may choose to replace them with tires having a lesser top speed rating. In situations where tires having different top speed ratings are mixed on a vehicle, the maximum speed certification is limited to the top speed certification of the tire with the lowest speed rating.
Speed ratings are indicated as follows:
Rating Maximum Speed
Q 159 KPH (99 MPH)
S 180 KPH (112 MPH)
T 189 KPH (118 MPH)
U 198 KPH (124 MPH)
H 208 KPH (130 MPH)
V Above 208 KPH (130 MPH)
(Without service description)
V 238 KPH (149 MPH)
(With service description)
Z Above 238 KPH (149 MPH)
Speed ratings do not indicate how well a tire handles or corners. They only certify a tire's ability to withstand high speed.
Light Truck Tires
Here is the typical information on the sidewall of a light truck tire:
"LT" stands for Light Truck
"LT235/85R16" is the size designation for a metric light truck tire
"LOAD RANGE D" identifies the load and inflation limits
"RADIAL" indicates that the tire has a radial construction
"MAX LOAD SINGLE 2623 lbs. AT 65 psi COLD" indicates the maximum load rating of the tire and corresponding minimum cold inflation pressure when used in a dual configuration
The other markings on the sidewall have the same meaning as described for the passenger car tire.