These numbers and letter together are called the service description. The numbers indicate the tyre’s maximum “load” rating, or the amount of weight the tyre can bear (“93” stands for 650 kilograms, or 1433 pounds), and the letter denotes the speed rating, or how fast the tyres can safely rotate (“W” means 270 km/h, or 168 mph). The lowest rating typically found on passenger-car tyres is “Q,” which means 99 mph. The highest, “Y,” is good for 186 mph, and when enclosed in parenthesis, as in “(93Y),” it means “in excess of” 186 mph. These values are determined by tyre-testing machines in a lab, and the decoded load rating is also listed elsewhere on the tyre.