The government wants us to be safe, so they're making sure that if you're going 125 mph you have the right tires on your car. Through actual testing of the tire, they rate each one according to how fast you can safely travel on a set of them. It's pretty straightforward when you check the chart. Each letter represents a maximum speed. Chances are your tires will be rated for much higher speeds than you'll ever go, but it's still a useful number. The faster a tire can go, the softer the rubber compound they use to make it (softer rubber grips dry pavement better), so the tire will wear out faster than a "slower" rated tire.
Speed Ratings
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
Z- 149mph and over
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