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When it comes to buying tires, construction is what counts

It takes a lot of knowledge to be a wise tire buyer these days. By far the most popular tire on new cars is the all-season radial, but the all-season radial is also gaining rapidly in the replacement-tire market. Last year, for example, 24 percent of all replacement tires were all-season radials.

Besides radials, the major tire types today are bias-ply and bias-belted. Construction is what matters because -- along with size, tread pattern, and rubber compound used -- construction determines a tire's gripping power, tread life, and handling and traction qualities. More important, it determines the quality you'll get for the money you spend.

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Keep in mind that cars are designed to give the best performance with certain types and sizes of tires. The wise practice in choosing a tire is to follow what your owner's manual recommends.

Never mix radials with other types of tires because of the major differences in their handling characteristics.

Bias and belted tires can be used together if similar tires are matched on the front or rear.

If you use two different sizes of tires, be sure they are not on the same axle.

To help the would-be buyer sort out the choices, here's a rundown on tire types, information on how to sort out sidewall markings, and an explanation of the tire-grading system now in use: Bias-ply tires

Bias-ply tires have two, four, or more plies, or layers, of rubber-coated synthetic cords (rayon, nylon, polyester, or other material), which cross from one bead (the inner edge of the tire's outer wall) to another at an angle of 35 or 40 degrees. These plies alternate in direction to provide sidewall strength.

The main advantage of bias tires is low cost; they range from $30 to $40 each and have a service life of 15,000 to 20,000 miles. Bias-belted tires

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Bias-belteds have the same basic bias construction, plus two or more cord strips (belts) that are wrapped around the perimeter of the tire between the body plies and the tread.

The belts not only strengthen the tire against impact and puncture damage, but also serve to strengthen the tread so that it has less tendency to squirm on the road. Key advantages are improved steering control, longer tread life, and increased fuel economy because of a reduction in rolling resistance.

Belted tires usually cost from $50 to $70 each and have a service life of 20,000 to 25,000 miles. Radial tires

Radials have body cords that extend from bead to bead, all pointing toward the tire's center line.

The cords are further wrapped with a protective steel or fiber-reinforced circumferential ``belt'' for strength and puncture resistance. This construction creates a more flexible, shock-absorbing sidewall and reduces friction generated where the tire meets the road.

The benefits are a more comfortable ride, improved handling, and superior gas mileage and tread life.

Radials range in price from $50 to $100 each and have a service life of 30,000 to 40,000 miles. Sidewall markings

Once you've determined the type of tire you want, you'll need to refer to the sidewall markings of your present tires to obtain the appropriate tire size for your car. This will be a metric number, such as P205/75R14, or a nonmetric number, such as FR-78-14.

If you have the nonmetric number, you'll need to obtain the metric equivalent, since most new tires these days are metrically sized. Most reputable tire centers have conversion charts to help, or you can just call and ask a salesman. You can then use the metric number exclusively.

The metric number contains information that a tire buyer should know. On a P205/75R14 tire, for example, ``P'' means ``passenger car''; ``205'' is the width (in millimeters) sidewall to sidewall; ``75'' is a ratio of the height to width (a 60-series tire is shorter and fatter while a 75-series tire is taller and thinner); ``R'' is for radial; and ``14'' is the wheel diameter in inches.

In addition, the sidewall of today's new tires provides other information (mostly in small print). The sidewall will tell you:

Whether the tire is a tube-type or tubeless.

The number of plies in the tire and sidewall and their composition. You may find: ``Tread: 1 ply polyester + 2 plies fiberglass. Sidewall: 1 ply polyester.''

The tire's load limits and maximum ``cold'' inflation. (``Cold'' tire inflation applies to the tire pressure when a vehicle has not been driven more than a mile after sitting for three hours or more.) For example: ``Max. Load 590 kg (1,301 lbs.)''

In all loading conditions (which refers to the weight of passengers, luggage, fuel, etc.), use of the maximum load pressure will result in the best tire performance and fuel economy.

The letters ``DOT,'' followed by a three-digit number. This indicates that the tire complies with the Department of Transportation safety standards, while the number is a code identifying the manufacturer and the location of the tire plant. Tire-grading system

The Uniform Tire Quality Grading System (UTQGS) rates tires for three characteristics: wet-weather traction, heat dissipation or resistance to high temperature, and tread wear. The system applies to all new passenger-car tires made or sold in the United States, except for winter tires.

The information can be used to compare tire brands for these three key performance features.

Based on UTQGS, all tire manufacturers now are required to test each type of tire they make on a government highway course under government guidelines. The test results are transformed into a code of one number and two letters, such as 120 B/C, which the manufacturer must mold onto each tire's sidewall. The number refers to tread life, while the two letters refer to traction and resistance to heat buildup.

The treadwear rating is based on a test of more than 7,000 miles. Tire condition at the end of the test is used to project when the tire will wear out. Ten points are awarded for each projected 3,000 miles of tread life. So tires that receive a 120 rating have a projected tread life of 36,000 miles.

How well tires stop on wet concrete and asphalt is determined by a traction test that rates tires with either an A, B, or C. ``A'' is the best rating.

A tire's ability to resist heat generation is measured by a temperature-resistance test. If a tire runs at 115 miles an hour for 30 minutes during the test, after running at lower speeds for four hours, it is graded ``A.'' If it passes at 100 to 110 miles an hour, but not higher, it is graded ``B.'' A ``C'' grade indicates the tire has passed the 85- to 95-m.p.h. tests.

A ``C'' rating is not considered bad. Tires rated ``C'' still meet the federal standard for use on all US highways.

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