Dimension designation for tyre

Updated 11/9/2020

Dimension designation for tyre

Designations for tyre dimension, load index and speed rating.

The car has an approval for the complete vehicle with certain combinations of wheel rims and tyres.

Designation of dimensions

All tyres have a dimension designation, such as: 235/45 R18 98 W.


Tyre width (mm)


Ratio between tyre wall height and tyre width (%)


Radial ply


Rim diameter in inches


Codes for the maximum permitted tyre load, tyre load index (LI)


Speed rating for maximum permitted speed, speed rating (SS). (In this case 270 km/h (168 mph).)

Load index

Each tyre has a certain capacity to carry a load, a load index (LI). The car's weight determines the load capacity required of the tyres.

Speed rating

Each tyre can withstand a certain maximum speed. Tyre speed rating, SS (Speed Symbol), must at least correspond with the car's top speed. The table below shows the maximum permitted speed for each speed rating (SS). The only exception to these regulations is winter tyresBoth those with metal studs and those without., where a lower speed rating may be used. If such a tyre is selected, the car must not be driven more quickly than the tyre is rated for. For example, cars with Q rating tyres must be driven at speeds not exceeding 160 km/h (100 mph). The road conditions and applicable road traffic rules determine how quickly the car can be driven, not the speed rating of the tyres.


The maximum permitted speed is specified in the table.


160 km/h (100 mph) (used only on winter tyres)


190 km/h (118 mph)


210 km/h (130 mph)


240 km/h (149 mph)


270 km/h (168 mph)


300 km/h (186 mph)


The lowest permitted load index (LI) and speed rating (SS) for the tyres for each respective engine variant are shown in the car's registration document. If a tyre with too low a load index or speed rating is used, it may overheat and be damaged.

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