Read any good tires lately?
They are round and black, and every car has them. So why is something so ubiquitous so misunderstood?
Reading a tire’s sidewall is not a black art.
Let’s start with the easy part. In big, bold letters you will find the manufacturer’s name and the name they have given the line of tires. By now, we are all too familiar with Firestone and its ATX tires. Other examples are Goodyear’s Wrangler, Uniroyal’s Tiger Paw and so on.
Also in big letters (and numbers) are the tire size, aspect ratio, basic construction, wheel diameter, load rating and speed rating.
Here is a simple explanation of the nomenclature:
When it comes to passenger tires, there are six systems including the P-metric, alpha-numeric, millimetric, European metric, ISO metric and numeric, now in use only on antique cars. But the vast majority of tires sold in the U.S. are P-metric.
The “P” stands for “passenger,” as in passenger car. This doesn’t mean that they are used exclusively on passenger cars, however, because many sport-utility vehicles such as the Ford Explorer and even some pickup trucks such as the Dodge Dakota are shod with P-metric tires.
If, instead of a “P” you find an “LT,” it means the tire is designed for a light truck and is often found on heavier-duty full-size pickups.
The numbers following the P give the width of the tire in millimeters. In the graphic accompanying this article, the “215” indicates that the tire is 215 millimeters wide at its widest point. This is usually referred to at the tire’s section width.
The slash means nothing and is there to keep the following numbers from bumping into the previous ones.
The second set of numbers (the “65” in our graphic) indicate the tire’s aspect ratio. This is the relationship of the tire’s height to its width–its section height to its section width. Section height is not spelled out–only the ratio of the height to the width, which in our sample tire is 65 percent as tall as it is wide.
In general, the higher the aspect ratio, the softer the tire will ride because the sidewall flexes more. Low-profile tires, with short sidewalls, tend to give a harsh ride, but better road feel and cornering control because their sidewalls flex less.
The next letter, the “R” on our tire, shows it is a radial design. Virtually all tires are radials, but bias tires still can be found–especially for racing–and they use the alpha-numeric system. Radial tires provide better fuel economy and are generally more durable than the bias-ply tires of the past.
The final number in the sequence gives the diameter of the wheel on which the tire will fit. Curiously, this number is given in inches, not millimeters. Our tire is designed for a 15-inch wheel. This is not the overall diameter of the wheel, but the diameter where the tire’s bead, of inner lip, contacts the inner ledge, or bead seat, of the wheel.
After a space, there is a number followed by a letter. The number is the load index (see box) and the letter is the tire’s speed rating (see box). In our graphic, the “89” means the tire’s load rating is 1,279 pounds (580 kilograms) and the “H” indicates that the speed rating is 130 m.p.h. (210 km/h).
Speed-rated tires do not make the car go any faster, but are an indication of how fast you can safely go on such tires. In general, you can figure that the higher the speed rating, the better the tire’s performance because it has a heftier construction.
But unless you expect to do some performance driving, you probably don’t need tires with the highest speed ratings such as W or Y. You can save money and be safe with an “H” rated tire–even if your car came with tires of a higher rating–though it may affect handling. Chances are, you won’t approach the limit of even a Z-rated tire, which will tolerate speeds in excess of 149 m.p.h.
Uniform Grading System Now, we are ready to investigate some of the smaller type covering such things as treadwear, traction and temperature resistance. These three criteria are part of the Uniform Tire Quality Grading System created by the U.S. Department of Transportation (DOT).
As DOT says: “It is not a safety rating and not a guarantee that a tire will last for a prescribed number of miles or perform in a certain way. It simply gives tire buyers additional information to combine with other considerations, such as price, brand loyalty and dealer recommendations.”
The treadwear rating helps you determine what kind of life you can expect from the tire compared to others from the same manufacturer. Note that we said “the same manufacturer.”
In this case, the tiremaker sets its own standards and chooses a tire against which it will compare all its other tires. This base tire gets a rating of 100. So, if a tire has a treadwear rating of 200, it should last about twice as long and a 300 treadwear rating longer still.
This is merely a guideline because tread life depends on how well you keep the tire inflated, the way you drive, how often you rotate the tires, the kinds of roads (rough or smooth) you travel on and other external factors. You cannot compare one tire maker’s treadwear rating to another’s.
But tires with a lower treadwear rating are usually made from a softer rubber compound and may provide better traction or handling.
The government guidelines for determining treadwear are based on running vehicles in 18 passes on a 400-mile test course, checking the tread wear every 800 miles until 7,200 miles are accumulated on the tires.
Traction ratings are stated using letters. The traction grades represent the tire’s ability to stop on wet pavement and are measured under controlled conditions on asphalt and concrete surfaces. It has nothing to do with a tire’s traction when cornering.
These are U.S. DOT prescribed tests and the ratings (from highest to lowest) are AA, A, B and C. If you are buying high-performance tires, you will probably also want an A or AA traction rating.
Temperature resistance, another DOT prescribed rating, represents the tire’s resistance to heat as well as its ability to dissipate heat and is tested under laboratory conditions. Again, from highest to lowest, the ratings are A, B and C.
These grades apply only when the tire is properly inflated and not overloaded. Sustained high temperatures can not only reduce a tire’s life, they also can lead to sudden and catastrophic tire failure. Excessive speed, overloading and underinflation exacerbate this.
Finally, we get to the small print near the center of the tire.
The construction components tell how many tread plies and sidewall plies the tire has as well as what they are made of.
A typical tire may have a tread of four plies–two steel belt and two polyester cord, while the sidewall may have two plies of nylon cord. The maximum load is usually spelled out in metric (kg) and customary (lbs) followed by the maximum inflation pressure, again in metric (kPa) and customary (psi) terms.
The type of construction (radial in our graphic) and design (tubeless) information also is usually molded into the rubber.
DOT, as we’ve said, stands for Department of Transportation, and the letters and numbers following it refer to the safety code and may include a code for the manufacturing plant as well as the date of manufacture.
You can’t use an Orphan Annie decoder ring, but you probably can get a tire dealer to look up the code in his reference books if you are desperate for this information.
Some, but not all, tires have additional marking. The letters “TPC” on the sidewall indicate that the tire meets General Motors’ tire performance criteria.
An “N” followed by a number (for example N-0, N-1, N-2) is Porsche’s code that the tire is approved for its cars.
If the tire meets the Rubber Manufacturers Association criteria for mud and snow, it may have markings like M+S, M/S or M&S.
Some tires, such as those on Corvettes, are uni-directional–designed for a particular rotational direction. Such tires may have an arrow pointing in the direction of (forward) rotation. Right and left tires cannot be swapped. Incidentally, Corvettes have different size tires front-to-rear, so rotating them periodically is out of the question.
Other tires may have a notation indicating which side should go toward the outside of the vehicle. These tires may, or may not, be uni-directional.
Virtually all tires have wear bars across their treads. Normally, you cannot see them until the tread wears down to 2/32-inch, which is the minimum safe tread depth. On the base of the sidewall, you will find several lines pointing to the locations of these wear bars.
Before performance tires leave the factory, they are inspected to determine the high point of the tire. (No tire is perfectly round.) At this point, the manufacturer will put a match-mounting dot. It may be a sticker or paint. Performance wheels are not perfectly round either and the wheel’s low point is usually marked. This is often where they drill the hole for the valve stem.
Ideally, the tire’s high spot should be located at the wheel’s low spot. Don’t believe people who tell you that the dot means the tire is a “second” or otherwise substandard.
Reading a tire may not be as exciting as a John Grisham novel, but it has its highlights.
SPEED RATING
Speed ratings do not follow an alphabetic sequence. And, you may not see tires with the “L” through “R” ratings. The following ratings are for the maximum safe speed for which the tire is designed. These designations are not government controlled so there may be slight variations among manufacturers.
Rating M.P.H.
L 75
M 81
N 87
P 94
Q 100
R 106
S. 112
T 118
U 124
H 130
V 130+
Z 149+
W 168+
Y 186+
%%
%%
Load Index
The load index indicates the tire’s load-carrying capacity at the tire’s maximum inflation. Most passenger-car tires fall between the indexes of 75 and 105, while P-metric tires used on light trucks and sport-utility vehicles usually have a higher load index. Remember, this is for each individual tire, and dual wheels, for instance, can carry twice the load.
Load index Load capacity
75 853 lb/387 kg
76 882 lb/400 kg
77 908 lb/412kg
78 937 lb/425 kg
79 963 lb/437 kg
80 992 lb/450 kg
81 1,019 lb/462 kg
82 1,047 lb/475 kg
83 1,074 lb/487 kg
84 1,102 lb/500 kg
85 1,135 lb/515kg
86 1,168 lb/530 kg
87 1,201 lb/545 kg
88 1,235 lb/560 kg
89 1,279 lb/580 kg
90 1,323 lb/600 kg
91 1,358 lb/615 kg
92 1,389 lb/630 kg
93 1,433 lb/650 kg
94 1,477 lb/670 kg
95 1,521 lb/690 kg
96 1,565 lb/710kg
97 1,609 lb/730 kg
98 1,653 lb/750 kg
99 1,709 lb/775 kg
100 1,764 lb/800 kg
101 1,819 lb/825 kg
102 1,874 lb/850 kg
103 1,929 lb/875 kg
104 1,934 lb/900 kg
105 2,039 lb/925 kg
106 2,094 lb/950 kg
107 2,149 lb/975 kg
108 2,205 lb/1,000 kg
109 2,271 lb/1,030 kg
110 2,337 lb/1,060 kg
111 2,403 lb/1,090 kg
112 2,469 lb/1,120 kg
113 2,535 lb/1,150 kg
114 2,601 lb/1,180kg
115 2,679 lb/1,215kg
%% %%
