Tire Facts Studies

We have come to the understanding that many do not truly comprehend the information that is on their tires; that is, what does the jargon on the sidewall really mean?   This is a situation that we would like to try to help remedy with this little primer.  As of the writing (2001) it is both accurate and relatively complete and we hope it is of benefit to our readers.

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.

We have divided the information into several areas to help you both navigate and go to the area that is most germane to your needs.  We hope you like it.    Finally, this discussion will pertain only to DOT (Department of Transportation) legal tires.

Sidewall Information:

Sidewall information is all of the 'facts' about your tires that 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 lets 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:

• Tread Wear
• Traction
• Temperature

Tread wear 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 tread wear number.  Conversely, if you have a passenger car that travels long distances at normal speeds, you want a higher tread wear capability.  However, there is always a trade off.  higher tread wear 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.

Yes, V,W,Y and Z are a bit vague in their descriptions.   However, for most performance cars any of these are perfectly sufficient.

Tire Pressure:

Tire pressure is probably the most critical aspect of tire safety and failure.  Most often the failure mode is over heating 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.

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.

Rarely we see 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.

UPGRADING TO "BIGGER" TIRES

       It is important to maintain approximately the SAME overall stock tire height when upgrading to 16" or 17" or even 18" rims and tires.  In reality, you are not putting on "BIGGER" tires.  You are putting on wider tires with a lower profile.  This is known as Plus Sizing.

PLUS SIZING

 

     Plus sizing is one of the easiest ways to achieve enhanced performance and improve the appearance of your vehicle.  In the example above, a 15X7 wheel with a 205/65/R15 tire is considered the O.E. (original equipment) size.  Converting to a plus 1 size would mean increasing the wheel diameter by 1" (16X7 (or 7.5 or 8)) and selecting an appropriate tire to fit (225/55/R16).  Likewise, moving to a plus 2 fitment would result in a 17X8 (or 9) wheel and a 245/45/R17 tire.  It is important to note that although the wheel diameter is increasing, the overall diameter, or height, of the tire remains consistent.   As a matter of fact, this method typically maintains the overall diameter of the tire within +/- .25 inches.  This is crucial to preserving the speedometer reading and gear ratio.

     A couple of things happen with the tire to increase performance when we begin to move into plus sizes. First, the tire is wider (section width) and therefore gives a larger footprint.  Second, the sidewall becomes shorter (aspect ratio) and more rigid.  The two added together give better lateral stability and increased steering response.

DETERMINING TIRE HEIGHT

     Tire Height = Rim Height + 2 (Section Height).  You will notice that tire sizes are measured in TWO units, millimeters and inches.  To keep things simple, it is best to convert to ONE unit.  I will use inches.  To convert millimeters to inches divide by 25.4 (Example: 205mm/25.4=8.07" or 8").  So, the stock tire is 8.07" wide.  To determine the section height, take the Aspect Ratio times the tire width.  On the stock tire, the Aspect Ratio is 65 or 65 percent of the width.  Therefore, the section height on the stock tire is 8.07" X .65 = 5.2455" or rounded off, 5.25 inches.  Remember, there are two section heights when determining the overall height of the tire.  The rim size is 15 inches.  Plugging these figures into the tire size formula, we have:

Stock Tire height = 15" + 2 (5.25") which equals 25.5 inches.

EXAMPLES OF SOME TIRE SIZES THAT YOU CAN UPGRADE TO
 

Tire Size			Width	Height
205	65	15	8.07	25.49
225	60	15	8.86	25.63
225	55	16	8.86	25.74
245	50	16	9.65	25.65
245	45	17	9.65	25.68
255	40	17	10.04	25.72
275	40	17	10.83	25.66
275	35	18	10.83	25.58

    You will notice that the overall heights of the examples are within a quarter inch of each other even though the rim size varies from 15 to 18 inches.  The table above is not all inclusive.  There are other sizes that can be utilized.  These are the more popular sizes when upgrading.

HOW "BIG" CAN I GO?

     I personally think the "ideal" size to upgrade to is 245/45/R17 on the V6 Mustang.  However, you can easily go "BIGGER."   Some people have upgraded to 255/40/R17 in front and 275/40/R17 in the rear without any problems of rubbing on the fender well.  Remember, that with two different sizes on a car, the tires CANNOT be rotated.  Also, remember that "BIGGER" also means more $$$$ for the rims and tires.  A 18' or 20" tire can easily cost in excess of $300 each, plus the cost of the rim.

Wheel Tize:

Many of you are also concerned with the relationship of tire width to wheel width and proper fitment. Here are some good general guidelines for common sizes.

Again, these are guidelines to use to gain optimum life of your tires, they are not absolute rules carved in stone. For instance, an 8" wide wheel and 12.50" wide tire is a very common setup. The relatively narrow rim (in comparison to tire width) is well protected from damage due to the bulging sidewalls. The downside is that the bulge of the tire also leads to premature wear at the center of the tire unless low tire pressures are maintained. On the flip side of the coin, you could run a 12.50" wide tire on a 12" wide rim if you really wanted to. With this setup, there is very little protection of the rim afforded by the sidewalls. Any rock, stump, rut, or even curb that you may hit risks damage to very expensive wheels and could very easily pop the bead resulting in a total blowout. Choosing the proper tire width in relation to rim size will prolong the life of your tires and quite possibly save a few unnecessary trips to the tire shop.