Speed index (SI) is one of the most important pieces of information on the sidewall (next to the size and load index). The load index is identified by a letter, A1–Y.
Speed index determines the maximum speed permitted for the given type of tyre. It must match the specification of the vehicle’s producer and the load index. Driving at a higher speed than the maximum indicated for the particular type of tyre may lead to a serious accident.
Fitting tyres with a lower speed index than is recommended by the manufacturer is not permitted. This applies in particular to summer tyres. Remember that the load index is carefully selected by the manufacturer. Please note that the speed index of the tyres you select cannot be more than 2 levels higher than recommended. If you change the load index, you must put an appropriate sticker of notification on your vehicle.
The table above shows the maximum speeds permitted for each SI rating.