How To Read The Sidewalls Of Your Tires

June 11, 2010

If tire manufacturers wanted to keep their customers in the dark about what is between them and the road they would have designed a system just like they have for labeling tires. If you are a tire aficionado you probably know this stuff, but we mere mortals might benefit from the drill.

One good thing, the information is all on the tire, right there on the sidewall (that’s the smooth part without the tread that is never supposed to contact the pavement). I told you that this was tire reading 101. So bear with me.

Let’s start with the part that you’re most familiar with - the numbers that contain the size. On the tire you will find something like P205/70R15. That letter may not always appear. It designates the tire type “P” for passenger car and “LT” for light truck. But that makes too much sense, so we’ll move onto the numbers which are a lot more oblique. The first number is the tire width from sidewall to sidewall measured in what else but millimeters. This tire is 205 millimeters wide.

The next number, in our example 70, is what the industry calls Aspect Ratio. I wrote a formula for this which is tire height/tire width = Aspect Ratio. However the tire height is not expressed anywhere on the tire so the formula is useless, unless it helps you to remember that the second number represents how large the height of the tire is when compared to its width. This tire’s height is 70 percent of its width. Who knows how many millimeters high this tire is?

The next two characters make perfect sense. The letter “R” is for the type of construction which is radial. You might come across a “B” for bias construction which is used on trailer tires and some equipment applications. These letters pertain to the way the tires were put together in terms of the layering of the plies. In a radial tire the layers go across the tire and a bias tire has layers that run diagonally. The last number is easy, it is the wheel diameter in what else but good old inches.

After the tire size comes the load index and the speed rating, expressed in something like 103T. The number part is the load index which would have to be converted to actual pounds via a chart. The higher the  load index number is the higher the actual load carrying capacity, for example an index of 103 has a capacity of 1929 pounds and a 93 index converts to 1433 pounds. The speed rating goes up as the letter rises in the alphabet a “T” rated tire recommended for use in family sedans has a maximum speed capability of 118 mph while a “Y” rated tire used on exotic sports cars has a maximum speed of 186 mph.  Of course, all the tire manufacturers warn against using tires at their maximum speed capacity since they are numbers established in a lab.

Next is the DOT (Department of Transportation) number. It can be up to 12 digits and the last four numbers designate the week and the year of its manufacture. Closer to the wheel you will find the maximum cold pressure and load marking but they are so small, that it’s easier to consult the tire pressure recommendations expressed on your door.

NHTSA established the Uniform Tire Quality Grade (UTQG) Standards which show up on the sidewalls of tires. It was meant to provide a way for consumers to compare tires by offering grades in temperature, treadwear and traction, but unfortunately the tire companies perform the tests and have been known to allow marketing to creep into their grading procedures.

So there you have it, tire nomenclature as it appears on your sidewalls, if you only could read it without needing to stand on your head.       


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