Tire Facts

What does the jargon on the sidewall really mean? The facts pertain to sidewall information and most of the information is radial tire specific as that is what is on most of today's street and performance tires. As of the writing (2017) it is both accurate and relatively complete. This information pertains only to DOT (Department of Transportation) legal tires.


Sidewall Information

Sidewall information contains all of the 'facts' about your tires and is printed on the tire sidewall.  The following will pertain only to passenger tires.

Let's take the following tire size as an example: P275/40 ZR17.   Well, what does this mean to you beyond tire size?  In fact, there is a lot of information just in this marking and more in the other markings too.  For now let's explore this example.

First the letter "P" means that this is a passenger car tire and "P metric" is our version of metric tire sizing.

The "275" is the width of the tire in millimeters.   To convert this to inches, simply divide the millimeters by 25.4.  This means that our 275mm tire is actually 10.8 inches wide.

The "40" is the ASPECT RATIO of the tire.  Aspect ratio is best expressed as the ratio of height to width of the tire.  In our example, the sidewall height is 40% of the width of the tire.  Generally speaking, any tire with an aspect ratio of 50 or less is considered a low profile tire, more on this later.

The "Z" is the SPEED RATING of the tire.  In this case a speed rating of Z is good for speeds up to and over 149mph.

The "R" indicates that the tire is constructed with radial plies.  A "B" would indicate a tire constructed with bias plies and a "D" would indicate a tire constructed with diagonal bias plies.

UTQG (Uniform Tire Quality Grade)

UTQG is simply a tire quality grading system and it covers three important areas: 

  • Treadwear
  • Traction
  • Temperature
Treadwear ratings are usually on a 'base' of 100 for wear.  A higher number indicates a greater mileage from the tire and a lower number indicates a lower mileage from the tire.  You should pick a tread wear based on several things beyond how long a tire will last.  For example, if you drive a performance vehicle and drive it hard, you should pick a very soft compound tire and it will usually have a low treadwear number.  Conversely, if you have a passenger car that travels long distances at normal speeds, you want a higher treadwear capability.  However, there is always a trade off.  higher treadwear capabilities usually offer lower handling abilities and in performance tire, a tendency to vulcanize well before their tread has worn down.  This creates potentially dangerous situation.

Traction is rated either A, B or C; with A being the best case traction and C being acceptable in traction but inferior to both A and B.  Traction test are often very misleading as it is, by actual testing, a measure of the tire's ability to stop in a " straight ahead" condition, on a wet surface of concrete or asphalt.  It has no testing for cornering or acceleration capabilities.  One may infer things about the latter but one may be incorrect in these inferences.

Temperature is the test of the tire's ability to withstand heat.   Like traction, temperature is rated A, B or C; with A being the best case of a tires' ability to withstand heat and C being acceptable but inferior to A and B.   Heat is a killer of tires and the largest offender is either under inflation of overloading of the tire.  Another offender is selecting a tire with an insufficient temperature rating for your specific needs.

Speed Ratings

Speed ratings indicate the safe top speed of a tire under perfect operating conditions.  These include optimum inflation, road surface and ambient temperatures.

Speed Ratings
Ratingmphkm/h
Ratingmphkm/h
P93150
H130210
Q99160
V149240
S112180
W168270
T118190
Y186300
U124200
Z149+240+

Tire Pressure

Tire pressure is probably the most critical aspect of tire safety and failure.  Most often the failure mode is overheating caused by under inflation.

It is extremely important to remember that your recommended tire pressure is a 'cold' pressure; that is, it is the pressure in the tire when the vehicle has been parked for several hours and is not in the sun.  The maximum recommended tire pressure is written on the sidewall of each tire.  Having said that, there are times when you may want to alter your tire pressures.  For liability reasons, we will not address these at all.

There are factors that cause your tire pressures to change; some of these we should address.  This first is ambient temperature and the second is load.

Most tires are filled with air, which is a gas that reacts to temperature changes.  For the most part, your tire pressure will increase / decrease about 1 psi per 10 degrees of temperature.  Other factors often forgotten are the road temperature, vehicle speed and load on the tire.  Drive your car on a black, asphalt road in the summer and the road is often well over 110 degrees on a sunny 85 - 90 degree day.  Increasing your vehicle speed increases the friction between the tire and the road; this also increases the tire pressure.  A fully loaded vehicle also increases the tire's temperature as it increases the friction between the tire and the road.  It is not uncommon to see pressure increases of 5 - 7 psi.

When checking the pressure of your tires, it is best to develop a routine.  That is, try to check them in a shaded area at the same time be it morning of evening and make sure that your check the pressure when the tires are 'cold'.  We try to check our tire pressure at least once a month and always before a highway trip or performance driving.

Load Ratings

Load ratings indicate the maximum load capacity each tire is designed to support. Like speed ratings, assume near perfect operating conditions to obtain the ratings listed in the table below.

Load Ratings
Load
Index
 Pounds Kilograms Load
Index
 Pounds Kilograms
70739335 991709775
71761345 1001764800
72783355 1011819825
73805365 1021874850
74827 375 1031929875
75853 387 1041984900
76882400 1052039925
77908412 1062094950
78937425 1072149975
79963437 10822051000
80992450 10922711030
811019462 11023371060
821047475 11124031090
831074487 11224691120
841102500 11325351150
851135515 11426011180
861168530 11526791215
 871201545 11627561250
881235560 11728331285
891279580 11829101320
901323600 11929981360
911356615 12030861400
921389630 12131971450
931433650 12233071500
941477670 12334171550
951521690 12435271600
961565710 12536381650
971609730 12637481700
981653750    

Contact Patch

Contact patch is probably the most misunderstood aspect of a tires' function. Basically it is the tires' foot print; or in the terms of an often-used phrase, it is where the rubber meets the road. 

Contact patch is directly related to tire size and shape. Tires with high aspect ration have long, narrow contact patches and low profile tires have wide, short contact patches. In the latter case, the wide patch combined with the short, slightly flexing sidewall; is responsible for superior handling, stability and traction. On wet roads, these same qualities may become a 'negative', as a wide tire tends to hydroplane much faster then a narrow tire. That is, they may tend to ride 'on top' of the water and not the road. This is often a disaster waiting to happen. Most tire companies take great pains to try to engineer water dispersing tread patterns into their low profile, performance tires. 

Vibration Problems

In new tires, most vibration problems are a function of tire balance. The exception to this is the new car with low profile tires that has been sitting for prolonged periods. If you have gotten this far in the web site you probably know what is causing your vibration. This phenomenon is often seen with imports as they sit in containers while crossing the ocean. 

Other causes of vibrations are wheels that are out of round or warped. Aluminum wheels are very prone to this problem and the wider the diameter of the wheel, the greater the tendency for this to occur. 

If you have a vibration problem we recommend the you seek out a Hunter model 9700-wheel balance system; they are diagnostics for most tire and wheel problems. We also recommend that when changing tires you select a system that does not touch the wheel while dismounting or mounting the tires. 

There will rarely be tread separation or belt separation unless the tire has been run in an under inflated mode; still, this does occur and the problem is first manifested as a vibration.

Vulcanization & Compounds

As noted elsewhere in this web site, low profile tires with high, speed ratings tend to vulcanize rather rapidly. This occurs at about twice the rate of a 'regular' tire. There are several reasons for this; the first is the compounds that must be used in most of these tires, they have a finite number of heat cycles and harden with each heating and cooling. Also, they will vulcanize with age. That is, you don't have to drive the car much for the tire to vulcanize; just let the tire get old and it will get hard. Tire vulcanization and its' dangers have been addressed elsewhere in this web site. 

The bottom line in performance tires is this; most people buy tires with a far higher tread life than necessary and the tires vulcanize well before they are worn out. This creates a dangerous situation as the vulcanized tire compromises traction while cornering, braking and accelerating. you would do well to consider the UTQG tread wear factor when you buy your next set of performance tires.

Tire Care

Short and sweet. Wash your tires with a good detergent and a brush and wash them every time you wash your car. Laundry detergent works very well. Do not put any silicone 'shine' product on your tires. If you use UV protecting agent, get one that is silicone free such as 303.