By Tony Backler, UCR Member, Photos by Randy Gananathan, Provinz EditorĀ (from July 2017 Provinz page 34)
When we check out a car, we often āoohā and āahā over its cabin and technical features. Likewise, we may pay critical attention to the performance and complexity of its engine. But we seldom give much thought to the specification of the tires. Perhaps we should, for as many auto writers have written: itās these four patches of rubber that give us the thrills of driving and protect us from misadventure. And, if we take a closer look at the tires, there is a lot that can be learned from them.
In fact, all basic information about the tire is branded right on its sidewall! So, in this brief primer, weāll get down to ātire levelā and will look at four specific bits of tire information in order to help you understand what each means.
ISO (METRIC) TIRE CODEĀ
On the sidewall, you will find an alpha-numeric code sequence like ā295/30ZR18ā or ā205/55R16ā. What do these numbers mean? The first three numbers before the slash (ā295ā in that first example, ā205ā in the second) denote the section width of the tire, expressed in millimeters. Itās the distance of the cross section between the inner and outer sidewalls. In practical terms, itās the width of the part of the tire thatās in contact with the road. Thus, a ā295ā section width gives you a contact patch nearly 12 inches wide.
The second set of numbers (ā30ā or ā55ā in my examples) denotes the aspect ratio of the tire. It is the ratio of the sidewallās section height to its section width. Translated, it indicates how much cushioning the tire will provide. A lower aspect ratio means a stiffer tire, giving better, quicker steering responseāwhile a tire with a higher ratio will have a taller sidewall, suited to absorbing more bumps. In my example, the ā55ā tire provides a more cushioned ride when compared to the ā30ā.
Next, youāll see one or more letters (āZRā and āRā in my examples). The letter āZā, if present, is a speed rating, and indicates that the tire is capable of withstanding constant speeds in excess of 240 km/h. āZā is the only speed rating that will appear within the tire code sequenceāother speed ratings are marked elsewhere on the sidewall (see below). The letter āRā indicates that the tire is of radial beltĀ construction (most are these days). You may also see āRFā, indicating that the tire is a self-supporting run-flat, or a āZRFā, indicating that it is a āZā-rated run-flat tire.
The last two numbers (ā18ā and ā16ā in my examples) indicate the rim diameter, given in inches. If youāre tire shopping, you have to make sure youāre looking for tires in a size matching your rim diameter!
SPEED RATINGĀ
As mentioned above, a tireās speed rating is listed separatelyāin whatās known as the service description area. This is typically located just to the right of the tire code and contains two numbers (the tireās load rating) followed by a single letter (the speed rating). Popular higher-speed passenger car tire speed rating designations include:
- S = 180 km/h (112 mph)
- H = 210 km/h (130 mph)
- V = 240 km/h (149 mph)
- Z = Over 240 km/h (over 149 mph)
- W = 270 km/h (168 mph)
- Y = 300 km/h (186 mph)
TIRE MANUFACTURE DATEĀ
The date of manufacture is a four-digit numerical code that usually appears in a frame, to the right of the DOTĀ stamping on the sidewall. The first two digits indicate the week of manufacture and the last two digits represent the year. For example, a ā2511ā stamping means the tire was manufactured in the 25th week of the year 2011.
With many tire specialists recommending that tires be replaced by the time they are seven or eight years old, date of manufacture is important to know, especially when considering the purchase of a used vehicle, or used tires. We need to remember that tire rubber weakens due to wear and tear, and due to exposure to ozone and sunlight. So, if the date on your tires reads āXX08ā, you really should consider their replacement, even if they appear just fine!
It is important to note that a used car safety inspection does NOT take tire age into consideration. So it doesnāt matter if the tire is ten years old, it will pass provided that it meets all other conditions.
UTQG TREADWEAR, TRACTION, ANDĀ TEMPERATURE GRADESĀ
The Uniform Tire Quality Grade standard (UTQG) area of the sidewall lists a passenger car tireās treadwear grade, traction grade, and temperature grade. The treadwear grade is represented as a number such as ā80ā, ā280ā or ā560ā, and is relative to a base score of 100, representing a tire wear-tested by the US National Highway Traffic Safety Administration, under controlled conditions. A grade of ā200ā would indicate that a tire is expected to last twice as long as the test tire, ā400ā four times as long, and ā80ā, for example, only 80 percent as long. Traction grades āAAā (highest), āAā, āBā, and āCā (lowest) are based on the tireās straight line wet coefficient of traction across a test surface. Finally, a temperature grade is given as āAā (best), āBā, or āCā depending on the tireās ability to dissipate or resist heat buildup when running at high speed.
Comparing grades can help you fit your car with tires that best suits your style and type of driving. Tire retailers often list the UTQG grades in the form ā560 A Aā. That example tells us the tire is durable, has a traction grade of āAā, and a temperature grade of āAā. Were a manufacture to offer the same tire with improved traction (going from āAā to āAAā, for example), typically we could expect the tireās durability to be compromised. The loss of durability is the price paid for softer, grippier rubber.
Hopefully, the above outline will have helped give you a greater appreciation for those four tires on your car and the role they play in providing enjoyment, performance, and safety while driving. To help them continue to serve you well, get down on your knees and check them out, on a regular basis. Keep your eye on them and they wonāt let you down.