Speed ratings are determined by indoor laboratory testing methods which measure high speed tire durability under controlled test conditions.
These test procedures do not take into account under inflation, tire damage, vehicle characteristics, or road conditions which can lead to sudden tire failure or loss of vehicle control at much lower speeds than indicated by the tires speed rating.
The validity of using speed rated tires in the U.S. is based on the idea that the tires top speed capability must at least equal the vehicles top speed capability, since it cannot be assumed that the driver will always observe the speed limit.
Recently the speed rating is being
referred to as the "performance rating" of the tire, since the higher
speed rated tires generally offer improved handling and maneuverability
compared to lower speed rated tires.