Tire speed ratings charts are available online at BridgestoneTire.com, MichelinMan.com and BlackCircles.com. The rating is given as a letter that corresponds to the maximum safe speed a tire is capable of. For example, a rating of S indicates the tire is capable of up to 112 miles per hour.
Most tires have their code displayed on the side. The code is a sequence of numbers and letters containing information about the tire's performance, such as its load-bearing capacity, dimensions and speed rating, which is usually the last letter in the series. The rating is the optimal top speed for a tire in good condition and does not take into account such factors as degradation and weather.