04.Tyre design and maintenance
How to read a tyre
Each tyre has standard markings that allow you to pick the right type for your car. It’s a confusing mix of letters and numbers, and of measurements in mm and inches.
Here’s what it all means, using the code for the tested size P205/65R15 94H and 95H:
P — stands for passenger tyre.
205 — this is the section width (in mm) when the tyre is inflated on the standard rim it's recommended for and not under load. The section width is the distance between the tyre's exterior sidewalls.
65 — this is a percentage describing the tyre’s profile or aspect ratio. It’s the ratio between the tyre’s section height (distance from the wheel bead seat to the top of the tyre) and its section width — in this case 65%.
R — stands for radial, which is the most common construction method for passenger car tyres.
15 — this refers to the diameter (in inches) of the rim the tyre should be fitted to.
95 is the load rating index and tells you the maximum weight one tyre can carry (in this case it means 690 kg). Other examples: 84 (500 kg), 86 (530 kg), 89 (580 kg), 92 (630 kg), 94 (670 kg).
H — this is the speed rating index and tells you the maximum speed the tyre can travel at (in this case 210 km/h). Other examples: S (180 km/h), T (190km/h), V (240 km/h), W (270 km/h).
How old is my tyre?
All tyres are stamped with the date of manufacture. You'll see this in two varieties: 3 digits for pre-2000, and 4 digits for after 2000.
Pre-2000: the first 2 digits stand for the week in the year and the last digit stands for the year. So a 3 digit of 078 stands for the seventh week in 1998.
After-2000: the first two digits stand for the week in the year and the last two digits stand for the year. So a 4 digit of 0209 stands for the second week in 2009.
Tread wear rating
According to the Australian Vehicle Standard, the date of manufacture is required and is normally a three or four digit code on the tyre sidewall (it can be on either side of the tyre). Check this before you purchase your tyre.
For tyres manufactured in years 2000 and above, the first two digits refer to the week of manufacture and the third digit is the year. For pre-2000 tyres, a four digit code is required, the first two digits again refer to week and the final two refer to year.
Although there is no recommended time period in the standard regarding the age of tyres, we often see retailers recommending five to six years for the replacement period of tyres. Tyre ageing will depend on the many variables of how it is used.
Other things to consider when purchasing a tyre are the signs of age such as micro splits in the tyre tread and sidewall that are visual up close, the result of oxidation of the tyre.
Tyre models that are marketed in the US have to have a tread wear rating as part of the Uniform Tyre Quality Grading System (UTQG) operated by the US government National Highway Traffic Safety Administration. About half the tested models have this rating.
Under the system, a tyre’s tread wear is measured under controlled conditions involving an 11,500 km drive on a specified test course, and compared to a ‘standard’ tyre with a rating of 100.
A rating of 200 indicates that the tested tyre should last twice as long as the standard model. So the higher the number, the longer you can expect a tyre to last.
However, the rating is comparative only. Real-life wear of a tyre depends on a number of aspects, such as the road surface, tyre pressure, wheel alignment and driving style.
Also part of the UTQG, and printed on the tyre sidewall next to the tread wear rating, are:
- A traction rating that grades the tyre’s wet braking traction (AA, A, B or C, with AA being best).
- A temperature rating, which indicates a tyre’s ability to dissipate heat (A, B, C, with A being best).
Some tyres are directional, which means they’re designed to be fitted to the car so their tread pattern faces a particular way (usually marked with an arrow on the sidewall). Fitting them on the wrong side may affect the car’s handling and reduce the tyre’s life.
They’re usually premium tyres. If you use such a model and don’t have a conventional tyre as a spare, be aware that a one-directional spare only fits one side of the car. If you have to use it on the wrong side, drive carefully and only until you can replace the damaged tyre.
Don’t confuse one-directional tyres with asymmetric models designed to be fitted to the rim so that a particular side (marked on the tyre’s sidewall) faces outwards. With these, the spare can replace any of the other tyres.
Some car models have an emergency space-saver (narrower) spare instead of a full-size one. If you have to use it, follow the instructions in your user manual. There’s likely to be a speed limitation, and you’re only supposed to drive on it for a short distance to get you home or to the nearest tyre fitter. Use over longer distances or at higher speeds may damage your car.
The right pressure
- Tyre pressure is measured in kilopascals (kPa) or pounds per square inch (psi).
- Keep your tyres inflated to the pressure recommended by the car manufacturer — usually shown on a sticker on the inside of the driver’s door frame, in the glovebox or on the petrol tank flap.
- The car manufacturer’s recommendations refer to the pressure when the tyre’s cold, not after you’ve been driving for some time.
- When driving with a heavy load, inflate your tyres to a higher pressure — check the sticker.
- Driving with under-inflated tyres uses more petrol, adversely affects the car’s handling, increases wear and may lead to tyre damage.
- Check the pressure regularly — for example, make it a habit each time you fill up with petrol.
- And don’t forget to check the spare’s air pressure when you do the other tyres. There’s nothing more embarrassing…
- Every so often, do a visual check for objects embedded in the tread, such as stones or glass.
- Check for uneven wear, which could indicate a problem with the car’s steering or suspension.
- Run your hands over the tread and sidewalls to identify any bubbles, cuts or cracks.
- Keep an eye on the tyres’ tread wear indicators, which show the minimum legal tread of 1.6 mm. The indicators are small bars spaced across the grooves of the tyre’s tread pattern. Replace your tyres when the tread level reaches the indicators — at the latest.
- Rotate the tyres regularly — for example, at every service.
- When putting on new tyres, have them balanced and a wheel alignment done.
This article last reviewed December 2008.