Mounting tyres with a speed index smaller than that of the original tyres or than that recommended by the vehicle's manufacturer is prohibited. However, it is completely acceptable to mount tyres with a higher speed index.

In winter, it is acceptable to mount tyres with a lower speed index (by one letter) than those mounted in the summer. For example, if the summer tyres have a speed index code of W, winter tyres with a code of V can be mounted, but not those with a code of H.

Manufacturers calculate these speed indexes using tyres that are in good condition.
These indexes do not apply if your tires are damaged (bulges or cuts), underinflated, overloaded, or repaired. Speed indexes have been determined on highways without speed limits in Germany or on closed-course test tracks. Depending on the indexes, tyres retain more or less heat, undergo varying degrees of deformation, and perform differently at high speeds. Be careful, even if the tyres you mount can perform well at high speeds, no tyre manufacturer recommends driving above the speed limits.

Note that for tyres with a speed index of "Z", the index is included in the tyre's description, which explains the "ZR" designation.