There's a lot to absorb when it comes to choosing new tires. Every tire has a variety of specs that affect the wheels it can be mounted on as well as its handling characteristics, appearance, and even durability. Although it is often overlooked, one of the most important characteristics of any tire is its speed rating. Speed rating on tires is given as a single letter ranging from M up to Z. Many tire retailers and installers will also list the maximum speed allowed by a particular rating in MPH along with the alphabetical rating code.
Note that if you are looking at a tire's size code (the string of alphanumeric characters found on the side of a tire), the speed rating will be the final letter that you see.
What Does a Tire's Speed Rating Mean?
Put simply, the speed rating of a tire is the maximum sustained speed that it is safe to operate that particular tire at. In other words, an S-rated tire can be operated at a sustained speed of up to 112mph. The NHTSA provides a complete listing of speed ratings that can be found here. When considering the speed ratings on a tire, keep a few important facts in mind:
What Determines a Tire's Speed Rating?
The speed rating of a tire is determined by testing the tire under laboratory conditions, but it is the actual construction of a tire that determines how well it performs at higher speeds. Tires that are specifically designed for high-performance vehicles will usually have much higher speed ratings. In some cases, these tires may be rated for speeds up to 149mph or even higher. Higher-performance tires also tend to have shorter lifespans and harsher rides, however, which can be an important consideration.
In general, it's important to understand that a tire's speed rating is reflective of that tire's performance characteristics. You should not assume that a tire with a higher speed rating will be more durable when driven at lower speeds. The opposite is often true as high-performance tires are not designed with a long lifespan as a primary goal.
Should You Care?
All modern car tires have speed ratings that far exceed the speed limit in most areas, so it's natural to wonder if you should even care. In most cases, the answer is "no." Sticking with a speed rating that is the same as the speed rating of your car's factory tires is usually a fairly safe bet. If you routinely take your car to track days, however, going with a higher speed rating is a good idea.