Corvette Glossary Of Terms
Z
Z-Rated Tire
Once known as the highest speed rating of any commercially produced tire, the “Z-Rated” Tire is still used today to designate tires for high-performance vehicles.
The speed rating denotes the maximum speed at which a tire is designed to be operated.   When the “Z” speed rated tire was introduced, it was universally recognized as qualifying its usage for speeds that were”unlimited”. Prior to 1991, tire speed ratings were shown inside the tire size, before the “R” character. The available codes were SR (112 mph, 180 km/h), HR (130 mph, 210 km/h), VR (in excess of 130 mph, 210 km/h) and ZR (in excess of 149 mph, 240 km/h).
Today, the ZR code is still in common use, though generally combined with a W or Y speed code. 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). In the European community, tires that are not fit for a car’s or motorcycle’s particular maximum speed are illegal to mount. The sole exception are M+S tires, where a warning sticker stating the allowed maximum speed must be placed within clear sight of the driver inside the vehicle. Some manufacturers will install a speed governor if a vehicle is ordered with tires rated below the vehicle’s maximum speed.
