When Z-rated tires first hit the market nobody thought they would need anything higher than 149 mph. They also never specified how far over 149 mph the tires could go. As cars kept getting faster, the industry introduced W and Y speed ratings. When a Z rating shows up in the tire designation (e.g., 235/50ZR16 91W) the Z signifies it’s capable of speeds over 149 and the W indicates a top speed of 168. If the tire is rated Y, then it’s good to go up to 186 mph. If the Y rating is in parentheses (with the load rating as shown above) then the top speed has been tested in excess of 186 mph. We clear?