What's in a letter?

Thursday, May 20, 2010 by Tucker Glenn
T, Z, W...These letters appear on a tire, but what do they mean? Speed ratings can be a little confusing.

I get many calls like this one: "I have an 08 BMW 328i and I need a Z-rated tire." To be honest, Z-rated used to mean a lot more than it does now. When speed ratings first came out, Z was thought to be the highest speed rating you could get, so tire manufacturers put them on every tire and it became cool to have Z-rated tires. Nowadays, with new technology, Z is just a stepping stone up to a (Y) rated tire.

Wait a minute!!! Y is higher than Z?? Yes it is! As you can see from the chart on this page, the speed rating letters don't exactly follow the alphabet. With certain vehicles now capable of running well over 150 mph, the need for higher speed ratings was necessary. Now, the top dog is (Y), which can travel in excess of 186 mph. Certain tires like the Michelin Pilot Sport PS2 are rated this high in some sizes.

But who goes that fast? Not many of us. So how do you make sense of what speed rating to use on your vehicle? The first place to look is on your vehicle's tire placard. It is usually found on the driver's side door frame. This will give you the recommended tire size, as well as the speed rating. This is the minimum requirement for your vehicle. It is not recommended to put a tire on that has a lower speed rating than the requirement. Not because you are ever going to be going that fast, but more for what the vehicle is expecting the tire to do at certain speeds and temperatures.

Based on your vehicle's minimum speed rating, you can feel safe using any tire that is rated above that speed rating. But your choice should depend on what you're looking for out of the tire. Tires with a higher speed rating tend to be a little stiffer, but provide better steering response when driven at higher speeds. Lower speed rated tires typically provide a more cushioned ride.

And, as always, if you have any questions, you can give us a call read articles about the technical side of tires available on our website.

Comments on What's in a letter?

Monday, July 29, 2013 by David:
Is it OK to have Z rated tyres on the rear and W on the front? The car is a 1997 Porsche 911 C4.
Monday, July 29, 2013 by Tucker:
David, it's likely that both tires are W rated. The way to tell is to look at the service description of the tire, indicated by a 2 digit number and letter. One may say 90W and the other 94W, but they're both W, and they both can be considered Z as well. A "Z" rating is purely optional for tire manufacturers these days, as it only vaguely describes speeds 149+mph.

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