Tire speed rating (speed symbol) and load index are very important factors that determine tire performance. The common name for the load index and speed symbol is "service description".
The tire load index and speed rating for light truck tires have some specificity compared to the passenger car tires.
The load index (Li) is numerical code, preceding the speed symbol, related to a tire's load carrying capabilities and it shows a tire's maximum load capacity. A tire with a higher load index indicates an increase in load capacity.
The light trucks utilize single and dual tires. Single wheel application (fitment) means two tires on the same axle. Dual wheel application (fitment) means two wheels on the both ends of same axle.
Dual tires on the non-steering axle are used to increase a load capacity and withstand stresses if one of two tires becomes flat or significantly underinflated. The remaining tire will carry the load for both tires.
All four tires on the rear axle must be equal - same model and tread depth.
Some light trucks contain two load indices in the service description. For example, for tire LT265/75R16 112/109 R, the load indices are 112/109, the first applies to the single tire fitment and the second refers to the dual fitment. The numeric load index code 112 represents the maximum load carrying capacity for single wheel use and 109 represents maximum load carrying capacity for the dual wheel use. The first number 112 denotes a tire's ability to carry approximately 2,470 lb (1120 kg). The second number 109 refers to the dual fitment, regarding to a tire's ability to carry approximately 2271 pounds (1030 kg).
For corresponding values see the load index table.
When the LT tires are fitted in the dual assembles the load capacity is reduced 9% of the single load capacity.
The values of load index vary for the passenger tires and the light truck tires, range usually from 70 to 110.
The speed symbol, also known as the speed rating, denotes the maximum speed at which the tire can carry the load indicated by load index (Li), under service conditions specified by a tire manufacturer.
In our example the speed symbol "R" denotes the maximum speed at which the tire LT 265/75 R16 112/109 R can carry the load indicated by the load index "112" (for single use) and the load index "109" (for dual use).
Tire speed ratings are defined in the laboratory test conditions related to performance on the road and it doesn't mean that a vehicle can be safely driven at maximum speed. The actual speed must be lower than the maximum speed for which the tire is rated since it depends of factors such as: inflation pressure, load and tire conditions, driving conditions (road, weather and vehicle conditions).
Speed ratings are not relevant if the tires are not regularly inflated, and if they are overloaded, worn out, damaged or modified.
Light trucks can't achieve as high speeds as passenger cars. The speed indexes used on a light truck tire are: N, P, Q, R, S, T and H. Please see Tire speed rating chart.
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