Speed Rating

A tyre's speed rating is a letter code that corresponds to a tyre's maximum safe operation speed.  Locate the speed rating letter on the sidewall of one of your tyres and compare it with the speed index table.  Verify the suggested rating from your vahicle's Owner's Manual.  You can always fit a tyre with a higher speed rating, but manufacturers recommend against fitting tyres with a speed rating below their specification.

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 tyres 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.