Tyres by the numbers

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Tyres by the numbers

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Tyres are safety critical components on a vehicle, so understanding the meaning of codes moulded into their sidewalls is helpful in interpreting specific characteristics, including speed ratings, load index and even their date of manufacture.

The sequence of numbers, letters and, in some cases, symbols, helps motorists to identify tyres that are properly specified for their vehicles – an important consideration in achieving the best balance between acceleration, braking and cornering and ride comfort, noise and traction.

Here’s an example of a sequence of numbers and letters you may find on the sidewall of a tyre – 205/55 R 16 91 V. Deciphering the code reveals:

  • The number 205 represents the section width of the tyre in millimeters, measured across the tread from sidewall to sidewall;
  • The number 55 represents the tyre’s aspect ratio, also sometimes called the profile height. It is expressed as a percentage of the tyre’s width. In our example, the sidewall height is 55% of the section width. The lower the figure, the lower the height of the sidewall and, in theory, the better the grip when cornering. On the downside, low profile tyres tend to offer a less comfortable ride and are noisier than counterparts which feature higher sidewalls.
  • The R signifies that the tyre is of radial ply construction, with cord plies positioned at a 90˚ angle to the direction of travel to add strength. The construction method is the norm for almost all passenger car tyres manufactured today, though a few are constructed using a diagonal build – designated by the letter D – or a belted build, B.
  • The number 16 indicates the diameter of the wheel rim in inches;
  • The number 91 indicates the maximum load weight the tyre has been designed to support when correctly inflated. By referring to the load index table we’ve published, you’ll see that the 91 rating equates to 615kg;
  • The letter V indicates the tyre’s speed rating, in this case 240km/h. Refer to the speed ratings table for information regarding other letters.

In a trend that is becoming increasingly popular, some tyres produced by major manufacturers may carry a code which indicates for which car brand they were originally developed, for example AO for Audi; MO for Mercedes-Benz; N1, N2, N3 or N4 for Porsche; VO for Volkswagen; MO1 for Mercedes-AMG and RO1 for Audi Quattro. BMW and Mini share a five-pointed star symbol, ☆.

If the tyre is a run flat, the words run flat may be moulded into the sidewall, or the technology may be signified by an abbreviation such as SSR (for self-supporting run flat); ROF (for run on flat); EMT (for extended mobility technology); RFT (for run flat technology); ZP (for zero pressure) or DSST (for Dunlop self-supporting technology).

High-performance tyres include XL or EXL to indicate that they are designed to cope with extra loads, while tyres with reinforced sidewalls include RF or RFD markings. FR is used to signify tyres with flanged ribs, which offer rim protection features. In the case of 4×4 tyres, M+S is short for Mud and Snow, referring to tyres that are suited to operating in slippery, slushy conditions.

Other encryptions which may be stamped into sidewalls include UTQG codes, which indicate the tyres have been rated according to United States’ Uniform Tyre Quality Grading standards with regard to tread wear, temperature resistance and traction; or ECE approval codes to show that they conform to standards set by the United Nations Economic Commission for Europe (UNECE).

Along with this data, sidewalls carry lots of other information. One important item is the DOT code, a legal requirement in many countries. DOT means the tyre meets or exceeds the US Department of Transportation’s safety standards.

It is usually accompanied by the manufacturer’s plant code number; the tyre size code number; a group of optional symbols used by the producer to authenticate the identity the brand; and the date of manufacture, indicated by a series of numbers which designate the week and year of build – for example 3616 means the tyre was manufactured in the 36th week of 2016.

It’s important to know the age of the tyres you purchase when replacing your used set. Even though the rubber may look new, over time it could have lost some of its properties, the rate of degradation influenced by storage methods and the amount of oxidization that has taken place. Unused tyres will become more brittle, weaker and less elastic through exposure to air, moisture, heat or sunlight.

As a rule of thumb, it is best to avoid buying tyres that are more than five years old – the period for which most major tyre manufacturers warrant their products against manufacturing and material defects.

Regardless of their age, tyres should be replaced if they show significant signs of crazing or cracking in the tread grooves or sidewall, or if bulges are apparent. All tyres on the vehicle, including the spare, should be inspected periodically to determine their suitability for service.

PASSENGER CAR TYRE LOAD RATINGS

Load Index 81 82 85 86 87 88 90 91 95 96
Max Load (kg) 462 475 515 530 545 560 600 615 690 710

 

PASSENGER TYRE SPEED RATINGS (KM/H)

Letter N P Q R S T U H V Z W Y
Speed 140 150 160 170 180 190 200 210 240 240+ 270 300

 

TYRE TIPS

  • Under- or over-inflated tyres are dangerous. Ensure your vehicle’s tyres are correctly inflated to the manufacturer’s recommended pressure, usually listed on a sticker located on the driver’s side door pillar, inside the vehicle’s fuel flap or in the vehicle’s handbook.
  • Have your vehicle’s tyre pressures checked weekly, preferably when casings are cold.
  • When carrying heavy loads or when journeys involve sustained high-speed travel, inflate your vehicles tyres to the highest pressure recommended by the manufacturer.
  • Rotate tyres every 10 000km. By doing so you will help to promote an even wear pattern which will in turn aid to prolong service life.
  • Similarly, have your vehicle’s wheels aligned every six months – or at 10 000km intervals – to help maintain even tyre wear and to promote safe handling and braking.

 

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