The speed symbol is made up of a single letter or an "A" and a number. It indicates the maximum speed at which the tire can carry a load corresponding to its Load Index.
Speed Ratings Chart
|E||43||70||Z||over 149||over 240|
|G||56||90||(W)||over 168||over 270|
|K||68||110||(Y)||over 186||over 300|
Tires with a speed rating higher than 186 mph (300 km/h) are indicated by a "Y" in parenthesis. The load rating is often included within the parenthesis, e.g. (86Y).
In many countries, the law requires that tires must be specified, and fitted, to exceed the maximum speed of the vehicle they are mounted on, with regards to their speed rating code (except for "Temporary Use" spare tires).