Each vehicle has a minimum number of admitted speed codes listed in the vehicle registration document next to the measurements.
Tyres with a different code than those listed in the vehicle registration document may also be mounted, but there are some restrictions:
Summer tyres can only be higher whereas winter tyres (M+S) can be lower (never below Q).
In the latter case, a label must be placed inside the passenger compartment where the driver can have a clear view and must clearly indicate the maximum speed limit admitted by the mounted tyres. Moreover, these tyres must be replaced within the term indicated by the seasonal regulation which requires for winter tyres with lower speed code with respect to that indicated in the vehicle registration document to be replaced after the 15 of April (by no later 15 of May) with tyres with speed code conforming to the vehicle registration document (replacement of winter tyres with speed code conforming or higher than the vehicle registration document is not mandatory.
Even if it is highly recommended in order to prevent a less suitable tyre from wearing too quickly with high temperature.
Below a summary table provided to understand the ratio between the letter and the maximum speed.
(you will also find it in the images on the detailed description of each product)