Understanding Tire Speed Ratings

Speed Rating

Speed ratings for tires were established to match the speed capability of tires with the top speed capability of the vehicles to which they are applied. Speed ratings are established in kilometers per hour and converted to miles per hour (which explains why speed ratings appear established at “unusual” mile per hour increments). Even though tire manufacturer’s can manufacture tires capable of high speeds, none recommend use in excess of legal speed limits. The maximum operating speed of a vehicle must be limited to the lowest speed rated tire on the vehicle.

How Speed Ratings are Determined

Speed ratings are based on laboratory tests where the tire is pressed against a large diameter metal drum to reflect its appropriate load. Then they are run at ever increasing speeds (in 6.2 mph steps in 10 minute increments) until the tire’s required speed has been met.

It is important to note that speed ratings only apply to tires that have not been damaged, altered, under-inflated or overloaded.  Additionally, most tire manufacturers maintain that a tire that has been cut or punctured no longer retains the tire manufacturer’s original speed rating, even after being repaired because the tire manufacturer can’t control the quality of the repair.

Deciphering a Speed Rating Symbols

Beginning in 1991, the speed symbol denoting a fixed maximum speed capability of new tires must be shown only in the speed rating portion of the tire’s service description, such as 225/50R16 89S.

The most common tire speed rating symbols, maximum speeds and typical applications are shown below:

Higher Speed Ratings

When Z-speed rated tires were first introduced, they were thought to reflect the highest tire speed rating that would ever be required, in excess of 149 mph. While Z-speed rated tires are capable of speeds in excess of 149 mph, how far above 149 mph was not identified. This caused the industry to add W- and Y-speed ratings to identify the tires that meet the needs of new, higher speed vehicles.

While a Z-speed rating often appears in the tire size designation of these tires (such as 225/50ZR16 91W) the Z in the size signifies a maximum speed capability in excess of 149 mph.  The W in the service description indicates the tire’s 168 mph, 270 km/h maximum speed. 

Most recently, when the Y-speed rating indicated in a service description is enclosed in parentheses.  285/35ZR19 (99Y)  The top speed of the tire has been tested in excess of 186 mph indicated by the service description below:

Over the years, tire speed ratings have evolved to better identify the tire’s capability.  Thus allowing drivers to match the speed of their tires with the top speed of their vehicle.