A “Z” rating can mean different things. While it nearly always means a high performance tire for high-performing sports cars, “Z” may actually appear in the middle of a tire’s size information. When it does, it represents either a maximum speed of more than 149 mph (240 kph) or 186 mph (300 kph). Which of those speeds it refers to will depend on the specific tire. Contact the manufacturer to get details on any Z-rated tires. Sports car owners are more likely to encounter this speed rating than owners of typical family sedans.

Always choose the speed rating that corresponds to your vehicle manufacturer’s specifications, and be sure all four tires have the same rating. Speed ratings are based on laboratory tests under specific, controlled conditions. While these tests relate to performance on the road under those conditions, remember that real-life driving is rarely identical to test conditions.

When Z-speed rated tires were first introduced, they were thought to reflect the highest tire speed rating that would ever be required, in excess of 240 km/h or 149 mph. While Z-speed rated tires are capable of speeds in excess of 149 mph, how far above 149 mph was not identified. That ultimately caused the automotive industry to add W- and Y-speed ratings to identify the tires that meet the needs of vehicles that have extremely high top-speed capabilities.

While a Z-speed rating still often appears in the tire size designation of these tires, such as 225/50ZR16 91W, the Z in the size signifies a maximum speed capability in excess of 149 mph, 240 km/h; the W in the service description indicates the tire’s 168 mph, 270 km/h maximum speed.

A tire rated with the letter “Z” in the tire size designation (i.e. P275/40ZR17 93W or 275/40ZR17 93Y) is the tire which have a maximum speed capability above 240 kph/149mph. Tires having a maximum speed capability above 300 km/h (186 mph) must have “Z” in the size designation and the service description must include Y in parenthesis (i.e. P275/40ZR17 (93Y)).