Speed ratings are as follows:
S: 112 mph
T: 118 mph
U: 124 mph
H: 130 mph
V: 149 mph
Z: Was the highest rating in the 1980s at 150 + mph
With the advent of new sports cars that well exceeded the 150 mph barrier, the tire industry adopted a few new speed ratings:
W: 168 mph
Y: 186 mph
Every tire that is produced is tested to see how fast it can be driven on before failure. If an automaker builds a car that is capable of speeds of 131 miles an hour, they absolutely need to install a tire that is capable of this speed. An H-rated tire that is known to be tested up to 130 mph, so the automaker would have to move up to the next speed rating (V).
While there are no specific state or federal laws that I am aware of regulating what aftermarket tire can be installed on your vehicle, there is still a liability to the tire companies that sell and install the tires. Moving to a lower speed rating may also have adverse affects on your car's handling.
We recommend that your maintain your car's original speed rating or move to a higher speed rating in search of better performance.
Read more about speed ratings. You can also search by vehicle to see the recommended tires and speed ratings for your car, light truck or SUV.