Tire sizes have letters in them, but they don't always have the same letters. You're probably familiar with the "P" or the "LT" that sometimes shows up at the beginning of the size, and the "R" right before the wheel size, for example P245/40R18 or LT315/70R17. But what does it mean when the tire says "P245/40ZR18"? For that, you need to know a little bit about speed ratings.
Speed ratings on tires are expressed in letters and are generally listed along with the load index for a tire after the size, for instance 245/40R18 93H. 93 is a load index and the "H" is the speed rating. Together, the load index and speed rating make up what is called the service description. Anything rated for more than 149 mph is considered a Z-speed rated tire and sometimes the manufacturer will stick the "Z" in the size right before the "R" to denote a Z-speed rated tire. Not all Z-speed rated tires have a Z listed on the sidewall in this manner because it is not mandatory. The service description is more specific about the speed rating, so putting the Z in the size is mainly just an aesthetic choice. W and Y speed ratings are both rated for higher speeds than 149 mph, 168 mph and 186 mph respectively, which makes them also Z-rated tires. V-speed rated tires are rated up to 149 mph, and since they aren't rated for over 149 mph, they are not Z-speed rated tires.
What about a tire that has the same name, same size but has a different speed rating? The Michelin Pilot Sport A/S 3 comes in two versions, the H- and V- speed rated versions and the W- and Y- speed rated versions. This separates the higher performing, Z- speed rated versions (W and Y) from the lower performance (H and V) versions. When you look at these tires in 245/40-18, you will see one version listed as 245/40ZR18 and two versions listed as 245/40R18. If you look closer, you'll see that the 245/40ZR18 has a service description of 97Y, which means it is a Z-speed rated tire, but more specifically, it is a Y-speed rated tire. The same tire with a V or H speed rating will not show a "Z" in the size or service description, just a 97V or a 97H.
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