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 load carrying capabilities and it shows the tire maximum load capacity. The 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.
Tire Load range - Tire Ply rating – Equivalency chart
Tire Load Range or Tire Ply Rating indicates the tire maximum load capacity at the maximum tire pressure(similar to the Tire Load index).
The Load range is a newer term than the Ply Rating and it's an attempt to express a radial tire carrying strength in old, bias-ply terms. Formerly, the term "Ply Rating" was used instead of the "Load Range" to denote the strength of a tire, indicated with the number of plies inside the tire(ply rating: 4,6,...). The stronger the tire, the higher its ply rating.
Load Range - Ply Rating equivalency chart
* Letters I and K are deliberately avoided.
Tires with the Load Range letters: B, C, D, E and F; correspond to tires with former 4, 6, 8, 10 and 12 ply ratings. This doesn't mean the tires with aforementioned letter ratings really have the identical number of plies as the corresponding tires with ply ratings, but their internal structure is stronger even with a lower number of plies. Today's light truck tires have only 2 or 3 body plies or one steel ply, but with the load capacity which was formerly declared with heavy duty ply ratings (10, 12 or 14 ply rated).
Current light truck tires mostly use the term "Load Range" that is marked on a tire with a letter, to denote the maximum load carrying capacity of the tire. The further along the letter is in the alphabet, the stronger the tire and the higher the inflation pressure it can sustain; consequently, the tire can carry a heavier load. For e.g., the tire with "Load Range E" can carry larger load than the tire with "Load Range B", even if they have the same nominal tire size. The Load Range is dependent on the tire inflation pressure(find "load/inflation table" in Data Books of a tire manufacturer).
Tire load capacity
Load range indicates the tire maximum load capacity(similar to the Load index), but it's not the only factor that affects the tire load capacity, another is the tire size i.e., the tire volume. So, the maximum tire load capacity is influenced by the tire size and the inflation pressure. A bigger tire(i.e., greater volume) can hold more air and can be rated for a higher load. Also, a given tire size at a higher inflation pressure results in the tire rated for the higher load.
Excerpt from the load/inflation chart.
- Two tires with different sizes have the same Load Range(C) at the same inflation pressure(50 psi), but different Load Capacities, because of their difference in the tire size(i.e., tire volume). The tire with a greater volume(195/75R14) has a greater load capacity(1435 lb).
- Two tires with the same tire size designation(195/75R14) can have two different Load Ranges(C or D), because they have different tire construction. The tire with the higher Load Range(D) has a stronger construction, hence, it can sustain higher inflation pressure(65 psi), consequently, has a greater load capacity(1710 lb).
It implies that for determining the tire maximum load capacity one must take into consideration the tire load range and the tire size(i.e., tire volume), because both features influence the amount of the air in the tire, hence the tire load capacity.
As a rule, for two tires with the same tire size(i.e., volume), a greater load carrying capacity has the tire with a higher Load Range, because it has a stronger construction and can operate on a higher inflation pressures.
For two tires with the same Load Range letter, the one with a greater tire size(i.e., volume) has a greater load carrying capacity.
Conveniently, there are Load/Inflation charts which are means for translating a Load Range of a tire into the tire's maximum load capacity(kg/lb) at the maximum tire pressure(psi/kPa) and the other way around, from the given load capacity to the recommended tire pressure for that load - for the specific tire size.
The load/inflation charts are related to a tire load and inflation standards. Also, tire manufacturers publish the charts in their Data Books.
Therefore, to use these charts, one should first establish a standard by which a tire is made(e.g., ETRTO(EU), TRA(US), etc.) and then find appropriate Load/Inflation chart relevant to the tire standard, that is also marked on the tire sidewall. Industry load and inflation standards are in a constant change, and the tyre manufacturers continually update information about their products to reflect these changes. Printed materials may not reflect the latest load and inflation standards.
MAX LOAD AT MAX PRESSURE Tire mark
The tire mark like this: "MAX LOAD SINGLE 1380 kg (3042 lbs) AT 550 kPa (80 psi) MAX PRESSURE COLD" (the load can be separately indicated for the single and dual fitment) encompasses the Load Index and the Load Range. It corresponds to the tire's maximum load carrying capacity at the specified maximum inflation pressure. You should never inflate a tire beyond its maximum inflation pressure.
The tire maximum load carrying capacity requires the tire maximum cold inflation pressure.
Notice: A tire maximum cold inflation pressure marked on the tire is not the same as the tire recommended presssure which can be found on the T.I.P. – the Tire information placard or in the Tire/Inflation charts for the given load and tire size. Read more in the article Tire pressure.
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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