Discoverer STT Pro
40X13.50R17LT 121Q C

Speed Rating:

The speed rating is a measure of what speed the tire can safely maintain for sustained periods of time. A higher speed rating will generally indicate that you will have better control and handling characteristics. Speed rating standards are established by the U.S. Department of Transportation (DOT). Manufacturers assign speed ratings to tires after extensive performance testing. On the sidewall of every tire, you’ll find one of these speed rating codes:

M – Up to 81 mph
N – Up to 87 mph
P – Up to 93 mph
Q – Up to 99 mph
R – Up to 106 mph
S – Up to 112 mph
T – Up to 118 mph
H – Up to 130 mph
V – Up to 149 mph
W – Up to 168 mph
Y – Up to 186 mph
Z – (See Below)

What does ‘Up to 99 mph’ really mean?
Tires with a Q speed rating can safely sustain speeds of up to 99 miles per hour. The tire may be able to reach higher speeds, but traveling at speeds higher than 99 mph for any length of time would be unsafe - the tire is not designed to handle it.

What is a Z Rating?
For tires having a maximum speed capability above 149 mph, a Z rating may appear in the size designation ...above 186 mph, a Z rating must appear in the size designation, including a Y speed symbol in brackets.

A Few Tips About Speed Ratings…
Never mix tires with different speed ratings, as this may cause serious handling problems.

We don’t recommend downgrading the speed rating of your tires, as the vehicle manufacturer has done extensive testing to determine which tires match the various driving characteristics of the vehicle. However, if you are looking for better handling, there is generally no problem in moving up to a higher speed-rated tire.

Most importantly: Put safety first. The speeds shown are test speeds; they are not recommended speeds.