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Learn To Read Your Tires

Posted in General, How To, Tips, Tires by Kurt Ernst | April 2nd, 2010 | 2 Responses |

The Pirelli Calender Girls: they know about tires

How many of you really understand the codes on the sidewall of your tire? If you do, you’ll be able to tell your tire’s size, how much weight it can carry, how fast it can go, whether or not it’s rated for mud & snow and even the date it was manufactured. If you’re a gear head, you’ll want to know as much about your tires as possible; if you’re just a commuter, then you may only need to know your tire’s size.

Starting with the tire’s size, we’ll take these one at a time and give an example of each, based on the Michelin Pilot Precedas my 2006 Mazda MX-5 wears:

Tire Size Description (205/45 R 17)

The first number, 205 is the width of the tire tread in millimeters. The second number, 45, is called the “aspect ratio”, and represents the sidewall height as a percentage of the tread’s width. In the case of our example, the tire is 45% of 225 millimeters, or 101.25 millimeters high. The R denotes radial tire construction, and the 17 is the wheel diameter in inches.

Service Description (84W)

The first number, 84, is the load rating, or how much weight a tire can carry. Unfortunately, there is no direct correlation between the code and the actual value; to determine the tire’s load rating, you need to look it up in a chart. A tire with a load rating of 84 can carry 1,102 pounds; four of these can support up to 4,408 pounds.

The letter code indicates a tire’s speed rating, which was easy to translate when there were only a handful of codes. Today, there are over 14, so I use the chart here.

Per the example, my W rated tires are approved for speeds up to 168 miles per hour, quite a bit faster than a mildly tuned MX-5 can go.

North American Load and Pressure Marking (MAX. LOAD 500 KG (1,100 pounds), MAX Pressure 351 kPa (51 PSI))

Not my tires

For those of us living in North America, Big Brother has made it easy: the tire’s load rating (described above) has to be printed on the sidewall. As you can see, the load rating agrees with the one encoded in the service description.

The inflation pressure shown is the maximum pressure that the tire is designed to handle; never, under any circumstances, inflate tires to this pressure. Follow the instructions in your owner’s manual or inside the driver’s door for the proper (cold) inflation pressure.

DOT Tire Identification Number (OCLL KJVX 3505)

This code, mandated by the U.S. Department of Transportation identifies the location of manufacturing, the tire size, manufacturer’s specifications and week and year of construction. I can’t help you on the location and manufacturer’s coding (but I’m sure it’s on the ‘net if you look hard enough), but the week and date are easy. The first two numbers of the last sequence, 35, indicate that my tire was manufactured during the 35th week of the year. The last two digits, 05, are the year. My tire was manufactured in late August or early September of 2005.

Tires built before 2000 use a three digit code, where the first two digits designate the week and the last one designates the year. A code of 018 indicates a tire built in January of 1998.

Why is this important? Because tires have a shelf life. As a general rule, avoid buying tires older than five years, as rubber compounds degrade with time. The older a tire is, the less likely it is to perform to the manufacturer’s original specifications.

UTQG Rating (Treadwear 240 Traction AA Temp A)

Again, not my tires...

The Uniform Tire Quality Grade rating was originated by the U.S. D.O.T. to give consumers a reference point when shopping for tires. Based on standardized testing, a tire with a treadwear rating of 240 will last 2.4 times as long as the tire used to set the benchmark. This may be misleading, as tire manufacturer’s often estimate the treadwear based on initial wear. You cannot say, with any accuracy, that a Michelin tire having a treadwear rating of 240 will last longer than a Goodyear tire with a treadwear of 200. Confusing? That’s what happens when the government tries to standardize consumer products.

The traction rating is only slightly more relevant. A tire with a traction rating of AA will measure greater than .54 g when braked on wet asphalt and greater than .41 g when braked on wet concrete; both tests are conducted at 40 mph. Does traction give an indication of how sticky a tire is in the corner? No, not at all. It doesn’t reference lateral loading and it doesn’t reference dry weather traction.

Temperature grade is redundant, since you already have separate (and more detailed) information on you tire’s speed capability. A temperature grade of A simply designates that the tire will be able to operate at speeds over 115 miles per hour without building up too much internal heat.

Mud & Snow Rating (M+S on sidewall)

The mountain & snowflake denote a winter tire

The Pilot Precedas on the MX-5 are summer only tires and don’t have an M+S rating; in other words, don’t even THINK about attempting to drive in snow or on icy roads with these tires. Summer tires have much different tread designs and are made from different rubber compounds than all season or winter tires. Sure, having two sets of tires gets expensive, but winter tires are the best way to ensure that you’ve got traction when you need it most.

So that about covers it – feel free to hit me with any questions you may have on tires for your ride.

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2 Responses

  1. AC Tony says:

    I can’t read my tires. Can I have one or more of those girls do it for me? A weekly basis would be great. Thanks in advance.

  2. Kurt says:

    No, but I can arrange for a toothless 65 year old stripper from Las Vegas to read you bad Russian poetry once per month…