It would be great if we could all buy the most expensive tyres for our cars every time we needed them, but for most of us that’s impossible. Most cars are used as simple commuting tools, driven by ordinary people who live on a tight budget.
That doesn’t imply we have to put up with rubbish that doesn’t work in the rain, or that we have to fit the cheapest retreads we can find that are downright dangerous and wear out much quicker than new tyres. It simply means that most new tyres perform perfectly adequately for the way most of us drive, so there’s no need to fit the most expensive rubber on offer.
When you buy a second-hand car, one of the first things you should do is check the tyres for wear and to see if they are, in fact, suitable for the vehicle. Check also to see how old they are — there are codes built into the sidewalls to tell you this. The materials used to manufacture tyres have a limited lifespan, and begin degenerating after a year or two, so if your tyres are more than five or six years old you should seriously consider replacing them.
So, what can a set of tyres tell you that’s important? Firstly, the tyres should be the right size for your car. Tyre sizes are marked on the sidewall — a typical budget family sedan would have, say, “185/65- SR14” tyres fitted as standard. The first number — “185” — tells you the overall width of the tyre in millimetres. The “/65” informs you of the aspect ratio of the tyre, or the depth of its sidewalls as a percentage of its width. In this case, the sidewall is 65% of 185mm, or 120,25mm, deep.
Low-profile tyres handle better than standard ones because there’s less sidewall flex, but they’re much more expensive and prone to sidewall damage from kerbs, potholes and so on. Also, unless a larger-diameter rim is also fitted to compensate for the difference in circumference, a low-profile tyre will lower your car’s overall gearing, which, for everyday use, is usually a bad thing. Remember, the smaller the aspect ratio, the wider the tyre in relationship to its height, and the smaller its circumference if fitted to the same size rim.
The letter that usually follows the tyre’s width and aspect ratio is very important, as it indicates the tyre’s speed rating — the highest speed at which it is designed to run continuously. The most common speed ratings are:
- P: 150kph;
- Q: 160kph;
- R: 170kph;
- S: 180kph;
- T: 190kph;
- U: 200kph;
- H: 210kph;
- V: 240kph;
- W: 270kph; and
- Z: 240kph.
Our sample tyre quoted above is rated “S”, meaning that it’s capable of cruising at up to 180kph. After the speed rating, you’ll usually find an “R”, telling you that the tyre is of radial ply construction, followed by the rim diameter — in this case, 14 inches. Why are the rim sizes given in inches while the other dimensions are all metric? Your guess is as good as mine, but that’s the way it works.
When were the tyres manufactured? Most carry codes on their sidewalls to tell you this, at the end of the DOT markings. Until the year 2000, the codes consisted of three digits, the first being the year of manufacture, and the next two the week in that year. Older tyres bearing the date code 717 were thus hopefully made in the 17th week of 1997, but they could also date back to 1987, 1977 or even earlier. Tyres built more recently use a four-digit code, with 0351, for instance, telling us that they were manufactured in late December 2003.
When buying new tyres, especially from discounters, check the date marks before doing the deal. If they’re already three years old and you accumulate mileage slowly, you could end up driving on apparently healthy tyres that have gone well past their use-by date in a couple of years’ time.
Tyres carry a wealth of other information on their sidewalls, not much of it of any real use to the motorist. What is worth checking, if the tyres you’re looking at have them, are the Uniform Quality Tyre Grading (UTQG) ratings. These purportedly rate the tyre according to wear, traction in the wet, and heat resistance.
Tyres are graded for wear compared to a norm of 100, meaning that a tyre stamped “400” should theoretically give four times the mileage of a standard “100” tyre under controlled conditions. Wet traction and heat resistance are rated from A to C, with A being best. Generally, soft tyres grip better but wear faster than hard rubber.
So, there you have it. The mysteries of tyre markings exposed. Read them, choose your tyres sensibly, look after them and respect their limitations and you should have many thousands of kilometres of safe motoring without spending a fortune on unsuitable rubber.