This rating system describes the top speed for which a tire is certified. It does not indicate the total performance capability of a tire. As manufacturers made more tires that fit into this category, it was necessary to better regulate performance at standardized speeds to ensure safety. Recently, W and Y limited speed symbols have been added as higher speed categories. Always consult the manufacturer for the maximum speed of Unlimited Z tires.
Speed rating is identified as a part of the tire's sizing or service description.
In the latest attempt to standardize tire designations, all ratings except unlimited ZR incorporate the speed symbol and load index as the tire's service description. Consult the tire manufacturer for maximum speed when there is no service description.