Be Smart About Tires Part 1: What Does the Writing on the Sidewall Mean?

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By Courtney Evans

Tire Purpose-Size-Type

Tire Purpose-Size-Type


At most times, your tires go largely ignored.  We hardly ever pay attention to them unless something goes wrong.  They usually get our attention when they are either flat or when they are the cause of our failed state inspection.  Tires are extremely important and their function should never be taken for granted.

Your tires are literally “where the rubber meets the road” and choosing the right tires at replacement time can be critical.  So, when the nice sales rep at “Tire World” makes a recommendation on what tires to purchase, you need to be ready with an informed answer.  Don’t just let’em sell you any old tire and certainly not the most expensive tire for your vehicle based on that recommendation.

In order to be an informed tire consumer (I know that’s what you want, right?), you need to know a little bit about tires.  You not only need to know a little bit more about tires, but you also need to be able to read and speak “tires”.  By this I mean, you need to know the difference between one tire and the next.  What’s its purpose?  What’s it good at?  What’s it not good at?  How long should it last?

Yes, that’s a lot of unanswered questions, but the answers are there, if you know how to read “tires”.  What we’re going to do in this essay is learn a new language.  Its not hard and I promise there’s no test “at the end of this chapter”.  My goal is to make you an informed consumer because informed consumers save money.

On the sidewall of every automotive tire there’s a written description.  I know that there’s a lot of information on that tire.  It looks like a confusing bunch of numbers and letters that don’t seem to mean much.  That is, not much to the untrained or undereducated eye, but those in the “know”, know.  There are 5 groups of information that you want to pay attention to.  There’s the Purpose/Size/Type group, the 3 T groups, and the Inflation/Load group.

 This group has the largest script or font size and will look something like this:  235/45ZR17 97W XL M+S.  Let’s break it into digestible pieces with each piece acting as a place holder.

“235″-  This number represents the tire’s width measured in millimeters, sidewall-to-sidewall.  The higher number , the wider the tire is.

“45″-  This number represents the aspect ratio.  The aspect ratio is the tire’s height to width measurement.  The height of the tire is measured from the inside diameter to the outside diameter.  In our example, the height of the tire is 45% of the width of the tire, so 235mm x 45%= 105.75 millimeters.  The lower this number is, the lower the tire’s profile.  Performance tires typically have a low profile.

“Z”- The Z location stands for the service group.  What is the intended service of the tire?  In our example, “Z” represents Performance tire.  Typically the service group letter is at the beginning of the Purpose/Size/Type group.  A “P” would represent Passenger car and “LT” stands for light truck.

“R”- This letter represents the tire’s construction type.  In this case it is a radial tire.  It could be “D” for diagonal or “B” for Bias Belt

“17″- This number represents the rim/ wheel measurement, or the inside diameter.

“97″-  This number stands for the Load Index.  The Load Index stipulates the maximum load or weight the tire can hold.

“W”- This letter represents the speed rating.  The higher the letter is in the alphabet, the higher speed the tire is rated for, though the letter “Y” has a faster speed rating than the letter “Z”.  There are other exceptions to the rule.

“XL”-  In this case it means Extra Load, or the tire supports a higher inflation than a standard tire.

“M+S”-  This combination stands for Mud and Snow.

Now there will be some variation between tires and across different manufacturers, but the place holders will be the same.  Many of the letters and numbers simply represent additional functions and features, but I think you get the idea.  the Purpose/Size/Type group represents the overall physical characteristics of the tire.

The 3 T’s

Treadwear Traction Temperature

Treadwear Traction Temperature

This group represents the functional characteristics of the tire.

Tread wear-  This rating is based on the wear rate of the tire while tested under controlled conditions.  The higher the number, the longer the tread will last.  Thus, a tire with a tread wear rating of 300 will last twice as long as a tire with a rating of 150.  Obviously, your driving habits, tire rotations, and wheel alignments will have a lot to do with the actual wear of a particular tire.

Traction-  This rating symbolizes the tires ability to stop under wet conditions.  The grades are, from highest to lowest, AA, A, B, and C.  The Traction rating only reflects straight ahead braking and not braking while turning.

Temperature-  This rating reflects how the tire resists and dissipates heat.  Excessive heat will decrease the life of the tire.  The grades from highest to lowest are A, B, and C.

The 3 T’s are based on tires that are mounted properly, have proper inflation, and are not overloaded.

Inflation/Load Group
Inflation-  This rating shows the maximum, safe tire inflation measured in pounds per square inch.

Load-  This shows the maximum load or weight that the tire can safely handle.  This number directly reflects the rating number in our Purpose/Size/Type group which is 97 in our example.

The Inflation/Load group is the what you need to look for while monitoring and maintaining the proper inflation of your tires.

Inflation Load Group

Inflation Load Group

Being able to read and speak “tires” is going to come in handy while shopping online for tires.  This skill is going to become even more valuable when you’re face to face with the sales rep from your local tire store.  After giving him/her your tire size you can say something like “I need a tire with a minimum traction rating of AA, a minimum tread wear of 400, with a minimum speed rating of H”.  In this case, you’ll get a tire with great wet traction, a long tire life, and a speed rating for faster than you’ll probably ever drive.

They won’t be able to just sell you any old tire because you’re informed and educated.

Who knew that learning another language could be so much fun!  That’s funny.  I think I’ve heard that somewhere before.

Anyway, in Part 2 we’ll dig just a little bit deeper and discuss what types of tires to buy.  There’s lots of choices.  Until next time.





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