The "speed/performance rating" is assigned by the U.S. government for meeting certain standards. Generally, the higher the speed/performance rating, the better performance the tire offers.
General safety rules:
1. You should not "downgrade" the speed/performance rating. You should keep the same speed rating as the original tires your vehicle came with or upgrade to a higher speed/performance rating. Downgrading to a lower speed/performance rating could result in poor performance and handling.
2. All four tires on your vehicle should have the same speed rating. Mixing can cause handling and performance problems.
Speed ratings are identified by a letter.
The letter refers to the maximum speed the tire should travel. Realize the speed/performance rating is not just speed. It is also performance and handling.
Q- Up to 100mph
R- Up to 106mph
S- Up to 112mph
T- Up to 118mph
U- Up to 124mph
H- Up to 130mph
V- Up to 149mph
W- Up to 168mph
Y- Up to 186mph
Some letters appear out of order because they were introduced at different times. For example, "Z" was originally the "high end" speed rating but as tires evolved, "W" and "Y" were introduced. The measurement is based on kilometers per hour, which is why the intervals appear strange.
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