Tires, Page 2 of 4
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Tire grading
See Figure 9 The Uniform Tire Quality Grading System (UTQG) took effect in April, 1979. Under this system the Department of Transportation requires each manufacturer to grade its tires and establish ratings for treadwear, traction, and temperature resistance. These tests are conducted independently by each manufacturer following government guidelines to assign values that represent a comparison between the tested tire and a control tire. While traction and temperature resistance ratings are specific performance levels, the treadwear ratings are assigned by the manufacturers following field testing and are most accurate when comparing tires of the same brand.
Figure 9 The UTQG rating is listed on the sidewall of all passenger car tires.
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Treadwear The treadwear grade is expressed by a number in multiples of 10-a higher number indicating a comparatively longer tread life. The number 100 is assigned as the standard of 30,000 mile (48,300 km) tread life on a test track under controlled conditions, and other numbers represent a percentage up or down from 100. As an example, a grade of 150 represents a tread life 50% greater than 30,000 miles (48,300 km), or 45,000 miles (72,400 km). Traction The test for traction involves towing a trailer mounted with the test tires over concrete and an asphalt course wetted with a controlled amount of water. As the brakes are slammed on, the tire's coefficient of friction is measured. An "A" grade means that its traction exceeds a predetermined standard on both courses. A "B" grade indicates that it exceeds a lower predetermined standard on both courses. If the tire can't make the "B" grade on either concrete or asphalt, it receives a "C" grade. In 1997 a new top rating of "AA" was introduced to indicate even greater wet braking traction. However, due to its newness, this grade will probably be applied initially to new tire lines as they are introduced and later to existing lines which excel in wet braking, but had been limited to the previous top grade of "A". Note that traction grades do not indicate wet cornering ability. The UTQG ratings are not required on winter or light truck sized tires. Temperature resistance The test for temperature resistance is to roll the tire against a large steel wheel at increasing speeds until the tire either is destroyed or achieves a grade of "A", indicating that it can survive a sustained run of 115 mph (185 km/h) at 95°F (35°C). A grade of "B" indicates endurance at 100 mph (160 km/h), and anything less, but still above the federal minimum tire safety standard, receives a grade of "C". Reading the tire sidewall
See Figure 10, 11, 12 and 13 The tire sidewall contains just about anything you would want to know about a tire, most of it required by federal law. The UTQG rating is listed on the sidewall indicating treadwear, traction and temperature ratings. Size-Tire width and diameter are identified on the sidewall as shown in the accompanying illustrations and tire size comparison chart. Up to 1978, tires were designated in the alpha-numeric system. To conform to standards, most tire manufacturers have converted to use the metric designation Maximum pressure and load-This is the maximum load the tire should carry when inflated at its maximum cold inflation pressure. Consult your owners manual or tire dealer for the recommended inflation pressure for your vehicle. Very seldom will the tires be inflated to their maximum pressure. Load range-The load range in the alpha-numeric designation is a letter indicating the number of plies at which the tire is rated. Tires using the metric labeling system are divided into two load ranges: - SL = Standard Load (35 psi max)
- XL= Extra load (41 psi max.)
Type of cord and number of plies-Each of these is dependent on the other and will vary with tire construction. Dot compliance-Since 1971, all tires are required to carry certain standard coded information, prefixed by DOT. This indicates that the tire conforms to U.S. Department of Transportation safety standards. The coded information also identifies the manufacturer, date of manufacture and other significant characteristics on the tire.
Figure 10 The side wall of a tire using the metric designations looks similar to this. Although the measurements are in metric, wheel diameter is given in inches. Load and pressure are given in both metric (kg/kPA) and English (lb./psi).
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Figure 11 The old alpha-numeric tire designation system shown here has been replaced by the metric designation.
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Figure 12 The alpha load range ratings replace the old bias ply rating.
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Load Range
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Replaces Ply-Rating
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A
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2
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B
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4
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C
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6
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D
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8
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E
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10
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Figure 13 The DOT code supplies identifying information about the tire.
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©1998 W. G. Nichols - Chilton's Easy Car Care
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