A frequently asked question that myself and other Tire Rack sales representatives receive daily is one concerning the speed ratings of tires and what speed ratings mean. While you likely will never encounter speeds beyond that of your tires' capabilities, it's important to understand everything else a speed rating can tell you when making your purchase decision.
Keep in mind that tires receive their speed rating based upon the amount of deflection in the sidewall and surrounding shoulders of the tire. A tire that is more stable at high speed will build up less heat than one with higher levels of deflection, and therefore be labeled with a higher speed rating. It is for this reason that stiffly constructed Max Performance Summer tires have a higher speed rating than softer riding Grand Touring All-Season tires.
High speed ratings are generally associated with improved handling, are generally stiffer riding and are commonly found on tires that are performance oriented in nature. Tires with lower speed ratings will feel more relaxed and give up a measure of performance and responsiveness in exchange for better ride comfort.
If you're a driver that is focusing on ride comfort above all else, find the minimum speed rating required by your vehicle's manufacturer and adhere closely to it. In contrast, if you are an aggressive driver or one with a focus on track performance, obtaining a higher speed rating is not only important for the higher terminal top speed of the tire, but also other performance attributes, such as turn-in response and on-center feel.
Common speed ratings in use today can be seen by taking a look at, "How to Read Speed Ratings, Load Index & Service Descriptions."