In our article Understanding Tire Sizes, some of the examples show a load and speed code listed at the end of the tire size, also known as the Service Description. For example, for the tire size P225/45R17 91V the last 3 characters "91V" indicate the Service Description (Load Rating and Speed Rating) of the tire.
With the exception of Z-Rated tires, all speed-rated tires manufactured since 1991 require a Service Description code. The first 2-3 digits of the code is a numerical value representing the tire's Load Rating, followed by a single character that represents the tire's Speed Rating.
In our example above, the load index of "91" means that the tire can carry approximately 1,356 pounds (615 kg). As you can see in the following table, the higher the tire's Load Rating, the greater the load it can carry.
| Load Index | Pounds (lbs) | Kilograms | Load Index | Pounds (lbs) | Kilograms | |
| 71 | 761 | 345 | 91 | 1356 | 615 | |
| 72 | 783 | 355 | 92 | 1389 | 630 | |
| 73 | 805 | 265 | 93 | 1433 | 650 | |
| 74 | 827 | 375 | 94 | 1477 | 670 | |
| 75 | 853 | 387 | 95 | 1521 | 690 | |
| 76 | 882 | 400 | 96 | 1565 | 710 | |
| 77 | 908 | 412 | 97 | 1609 | 730 | |
| 78 | 937 | 425 | 98 | 1653 | 750 | |
| 79 | 963 | 437 | 99 | 1709 | 775 | |
| 80 | 992 | 450 | 100 | 1764 | 800 | |
| 81 | 1019 | 462 | 101 | 1819 | 825 | |
| 82 | 1047 | 475 | 102 | 1874 | 850 | |
| 83 | 1074 | 487 | 103 | 1929 | 875 | |
| 84 | 1102 | 500 | 104 | 1984 | 900 | |
| 85 | 1135 | 515 | 105 | 2039 | 925 | |
| 86 | 1168 | 530 | 106 | 2094 | 950 | |
| 87 | 1201 | 545 | 107 | 2149 | 975 | |
| 88 | 1235 | 560 | 108 | 2205 | 1000 | |
| 89 | 1279 | 580 | 109 | 2271 | 1030 | |
| 90 | 1323 | 600 | 110 | 2337 | 1060 |
Speed Ratings were established so that tires could be correctly matched to the top speeds of vehicles. If you happen to be driving on the Autobahn in Germany, there are no speed limits and driving at high speeds is legal. However, in the United States, we are legally bound by designated speed limits, so it is unlikely that you will ever achieve top speeds for your tires. Manufacturers also do not recommend using their tires in excess of the legal limits, even though the tires are engineered and constructed to be capable of high speeds.
The Speed Rating for tires is derived based on laboratory tests. Tires are mounted in contact with a large drum that simulates driving load and the speed is increased by 6.2 mph (10 km/hr) increments every 10 minutes until the maximum speed is reached.
Speed Ratings only apply to tires that have never been damaged, cut, punctured, under-inflated, over-inflated, or altered in any other way. Most tire manufacturers maintain that the tire's Speed Rating is no longer valid if the tire has been repaired due to a cut or puncture because the quality of the repair cannot be controlled or verified by the manufacturer.
The Speed Rating is a single character code that describes the maximum speed of the tire:
| Speed Rating | Miles/Hour | Km/Hour | Application |
| M | 81 mph | 130 km/h | |
| N | 87 mph | 140 km/h | Temporary Spare Tires |
| P | 93 mph | 150 km/h | |
| Q | 99 mph | 160 km/h | Studless and Studded Winter Tires |
| R | 106 mph | 170 km/h | Heavy Duty Light Truck Tires |
| S | 112 mph | 180 km/h | Family Sedans & Vans |
| T | 118 mph | 190 km/h | Family Sedans & Vans |
| U | 124 mph | 200 km/h | |
| H | 130 mph | 210 km/h | Sport Sedans & Coupes |
| V | 149 mph | 240 km/h | Sport Sedans, Coupes & Sports Cars |
| W | 168 mph | 270 km/h | Exotic Sports Cars |
| Y | 186 mph | 300 km/h | Exotic Sports Cars |
| Z | 149+ mph | 240+ km/h | Sports Cars |
Over the years, the Speed Rating symbols were marked on tires in a variety of ways. Many early tires had the Speed Rating included within the tire sizing information. For example:
However, since 1991, the speed symbol has been required to be shown only in the Speed Rating portion of the tire's Service Description (example 225/45R17 91V) to denote the tire's fixed maximum speed capability. The exception to this rule occurs with Z-rated tires. The Z Speed Rating often appears within the tire size (example 225/45ZR17) and signifies a maximum speed in excess of 149 mph.
When Z-rated tires were first introduced, it was thought that the maximum speed of 149+ mph (240+ km/hr) would be sufficient to describe the top speeds of the tires. However, it later became apparent that the Z-rated tires did not specify an upper bound beyond 149 mph. Hence, in order to accommodate new vehicles with extremely high speed capabilities, the automotive industry created the W and Y ratings to specify the top speeds of 168 mph and 186 mph, respectively.
Most recently, Z-rated tires may also show a W or Y rating in the Service Description. For example:
In the first example above, the Z-rating indicates that the tire is rated for 149+ mph. In the second example, the 90W indicates that the tire is rated for 149+ mph with a maximum speed rating of 168 mph (270 km/hr). Similarly, the 94Y in the third example indicates a tire rated for 149+ mph with a maximum speed of 186 mph (300 km/hr). In the fourth example, the (92Y) enclosed in parentheses indicates that the tire has been tested at speeds in excess of 186 mph.