Mounting tires with a speed index inferior to the one of the original tires or than that recommended by the vehicle's manufacturer is prohibited. However, it is completely acceptable to mount tires with a higher speed index.
In winter, it is acceptable to mount tires with a lower speed index (by one letter) than those mounted in the summer. For example, if the summer tires have a speed index code of W, winter tires with a code of V can be mounted, but not those with a code of H.
Manufacturers calculate these speed indexes using tires 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, tires retain more or less heat, undergo varying degrees of deformation, and perform differently at high speeds. Be careful, even if the tires you mount can perform well at high speeds, no tire manufacturer recommends driving above the speed limits.
Note that for tires with a speed index of "Z", the index is included in the tire's description, which explains the "ZR" designation.
