Z just means they are rated for a minimum of 149 mph. Because modern sports cars can do that without blinking, the tire manufacturers actually have a W and a Y rating attributed to their sports car tires. Michelin adds a Y (the highest rating) to their tires, meaning they are rated for 186+ mph. This is usually found on tire spec sheets, as the tires usually follow tradition and are just stamped with "ZR" on the sidewall.