The letters ZR on the sidewall of a tire refer to the tires speed rating. Historically, the speed rating ZR on a tire meant that the construction of the tire could reach 150 miles or 240 kilometers per hour. Today, a tire labeled ZR has more capabilities than before and can be classified into three different speed ratings, which can be either W, Y, or Z. The ZR rating is for tires with a speed capability above 149 mph. ZR is initially divided into W, Y, and Z, and the tires can travel up to 149 mph under all three of these speed ratings. The ZR label is still present in the catalogs of shops and tire manufacturers, but it is no longer relevant. The letter R in a tire rating stands for radial construction, which is the typical form of tire construction used on most passenger vehicles nowadays. Therefore, the "ZR" is the speed rating, while the "R" refers to the type of tire.