Reading our Tires

Published in the February 2010 issue of “Die Porsche Kassette”

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There is a lot of useful information included on the sidewall of our tires.

We’ll try to simplify it by taking it one part at a time.


Tires actually have a shelf life.  It is recommended that tires over 6 years old be removed from use.

Obviously, tires that are subject to permanent UV damage from the sun can deteriorate sooner than those that are protected from the sun’s rays.

The U.S. Department of Transportation (DOT) in particular the National Highway Traffic Safety Administration (NHTSA) has required tire manufacturer’s to stamp the date of manufacture right on the tire’s sidewall, but they’ve allowed them to code that information.

Prior to 2000 manufacturers could stamp the code on the inside of the tire, but the NHTSA mandated a few years ago that the code be moved to the outside of the tire, allowing the manufacturers to phase in this information with a deadline of 2009.

Since 2000 the last four digits of the TIN (Tire Identification Number) includes the week (first 2 digits) and year of manufacture (last 2 digits).

Let’s look at the following TIN:  DOT  H2LF YA9J 3507

The last four digits 3507 tells us that this particular tire was manufactured in the 35th week of the year 2007.

Prior to the year 2000, only three digits were used.  The first 2 were for the week, but only one digit was for the year, so there’s no way to determine from what decade that last digit is from.

In this TIN: DOT EJ8J DFM 408

The last 3 digits 408 tells us that the tire was manufactured in the 40th week of 1998 (or 1988, 1978, 1968...).


On the sidewall, you will also find tire’s Service Description which identifies the Load Index and the Speed Rating.

Let’s study the following example: 225/40ZR17 94Y

The first part corresponds to the tire’s physical size (more on that later) and the last grouping corresponds to the Load Index and Speed Rating.

The 94 corresponds to it’s load carrying capability.  In this particular case an Index of 94 corresponds to a load of 1477 lbs (670 Kg).  The higher the Load Index (LI) number, the higher  the tire’s load capacity is.

The typical Load Indexes for passenger cars range from 70 to 110 (following is a sample):

The “Y” is the Speed Rating, which in this case signifies that the tire’s maximum speed is186 mph (300 kph).

If this Speed Rating were in parentheses) i.e.: (94Y) it would signify that the tire has been tested at speeds in excess of 186 mph (300 kph).

When Z-speed ratings first appeared on tires, it was thought to reflect the highest tire speed rating that would ever be required, in excess of 149 mph.  Because high performance automobiles keep getting faster and faster, the industry added W- and Y- speed ratings to indicate the tire’s maximum speed:

W = 168 mph (270 kph)   and    Y= 186 mph (300 kph).

Elsewhere on the sidewall there is still more information, such as the Uniform Tire Quality Grade (UTQG) Standards.  These Standards spell out the Treadwear, Traction and Temperature (Resistance) Grades.

TREADWEAR: Is a relative number because it indicates the relative wear compared to a test tire. 

What’s important is that the higher the Treadwear number the longer lasting the tread will be.

TRACTION: Indicates the tire’s straight line wet coefficient of traction.  This grade does not evaluate braking, or cornering in dry conditions, wet cornering or high speed hydroplaning resistance.

The different traction grades are:

SIZE:  The first segment (255) is the width of the tire in millimeters (10.04 in).  The second segment (40) is the height of the tire (sidewall) as a percentage of its width.  In other words, in this case, 40% of 255 mm = 102 mm (4.02 in).  The “Z” is the speed rating and the “R” corresponds to the construction type (Radial). The “94Y” has been covered above.  Additional markings, such as Type of Construction, Country of Origin and other are also to be found on the sidewall.

ROTATION:  Many of today’s high-speed tires have the rotation marked as well.  Either by indicating the direction of rotation or by the word “OUTSIDE” which indicates that that part of the tire should face to the outside.

Many of today’s tires are also asymmetrical and unidirectional, meaning that the tread pattern is different side-to-side and that they are meant to rotate in one direction only.

This means that they cannot be transferred from the left side of the vehicle to the right side or viceversa.  In our modern Porsches it also means that they cannot be transferred front-to-back because the rears are generally wider than the fronts.

So, in order to have our tires last as long as possible, and because they cannot be rotated, as in conventional vehicles, it is extremely important to have the tires properly balanced, properly inflated and the car properly aligned.  But that will be matter for another Tech Article.

You will note that we did not include INFLATION in this article.  Maximum tire inflation is generally indicated on the sidewall as well, but it is a maximum number for structural integrity purposes.  The correct inflation pressures for each vehicle are indicated by the car’s manufacturers on a sticker generally found on the door or door jamb of the vehicle.

For more information on tires, please feel free to visit my web pages at

Happy Porsche’ing,

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