Why they decided to encrypt these ratings instead of just printing the actual number is anyone's guess. Since the speed rating is a letter (for example, a speed rating of 'S' means that the tire can handle a maximum speed of up to 112 miles per hour/180 kilometers per hour), you'll have to check out a chart to learn what speed each of letters indicates. The speed rating is represented by a letter and tells you the maximum speed at which this tire can go, assuming the car isn't carrying a load that exceeds the tire's load rating. The second piece of information that makes up your tire's service description is its speed rating. The tire's load rating is considered part of the markings on your tire that make up its service description. In addition, your tire also has a different marker on it that tells you the load rating at a given inflation pressure. A tires load index designates the maximum load carrying capacity of a tire at maximum air pressure and speed. For example, a tire-load rating of 105 means that these tires can handle a maximum-weigh capacity of up to 2,039 pounds (925 kilograms). However, do note that the tire load rating number itself is not the actual weight limit. The higher the tire-load rating, the more weight your tire can handle. Your tire's load rating tells you the maximum weight capacity that your tire is designed to handle. A tire that attains a traction index equal to or greater than 110, compared to the ASTM E1136-93 (re-approved 2003, incorporated by reference, see §571.5) Standard Reference Test Tire when using the snow traction test as described in ASTM F1805-00 (incorporated by reference, see §571.5), and that is marked with an Alpine Symbol specified in S5.

Tire load index