Johannesburg - Most people rely on a tyre's tread depth to determine its condition - but that's not the only criterion.
Motor Industry Workshop Association chairman Les McMaster pointed out that all rubber compounds - including those used in automotive tyres - deteriorate over time, regardless of the condition of the tread.
"An old tyre can be just as hazardous as a tyre with worn tread," he said.
For most motorists, it's not an issue.
McMaster said: "If you drive more than about 20 000km a year the tread will wear out in three or four years, long before the rubber does.
SMALL CRACKS
"But if you drive only short distances, or have a car that you drive only on weekends, tyre deterioration can become a problem - and that also applies to spare tyres and 'new old stock' tyres made years ago that have never been used."
Over time, tyre rubber hardens and small cracks appear on the surface and inside the tyre. Eventually these cracks reach the steel or fabric belts that give the tyre its shape, causing the rubber to separate from the belts - and at that point a blowout becomes a matter of when, not if.
What's worse is that the exaggerated flexing and extra heat caused by running the tyre while underinflated will accelerate the process.
"Some tyre manufacturers insist their tyres will last at least 10 years," said McMaster, "but there's no way to predict how long a tyre will last because factors such as heat, storage and conditions of use reduce the life of a tyre."
SO HOW DO YOU CHECK THE AGE OF A TYRE?
Every tyre has a DOT-code on the inside - three digits on a tyre made before 2000 (which you shouldn't be using anyway) and four on a tyre made since 2000. The first two digits indicate the production week and the second two the year.
McMaster pointed out that South Africa's hot, sunny climate contributes to the aging of our tyres - spare tyres included.
"On a bakkie the spare is usually mounted underneath, exposed to the elements - and if it's in the boot, it's baking in a miniature oven all day. Even a tyre that's never been fitted on to a rim deteriorates over time; it does so more slowly but it ages nonetheless."
"Avoid buying used tyres," he advised, "and check the date on new tyres before the fitment centre staff mount them on your rims.
"Just because a tyre looks new doesn't mean it hasn't been on the shelf for years, aging while it waits."