Speed index is one of the most important tire parameters, that every driver of a motor vehicle has to adhere to. It is indicated by the abbreviation “SI” (eng. Speed Index). It tells us the speed permitted for driving on a given tire. This index is located, along with other markings, on the tire’s sidewall in a form of letters. Each letter stands for a different value, which is described in the following table:
Incompatibility with the value of the load index is dangerous. As a result of malfunctioning tire with too heavy load, damage or destruction of tire can be caused, which may result in a traffic accident. Additionally, in case of tires with the highest speed index (from “V” up), the driver is obliged to reduce the maximum permissible load in the car according to the principle:
Index „V” – 3% every 10km/h between 210-240 km/h
Index „V” – 5% every 10km/h between 240-270 km/h
Index „Y” – 5% every 10km/h between 270-300 km/h
Keep in mind that each manufacturer matches the appropriate speed index to the car on the basis of its technical data. In case of summer tires drivers need to strictly stick to it, while in the case of winter tires index light correction is allowable. This is due to the fact that during winter we usually drive slower due to worse road conditions.