Those round, black, rubbery things on which our cars roll along are often taken for granted, yet they are as scientifically designed and precision- engineered as the car's engine.

We are talking about tyres, which have come a long way since John Boyd Dunlop invented the world's first practical pneumatic tyre in 1888.

Today more than a billion tyres are produced yearly worldwide, with the top three companies, Bridgestone, Michelin and Goodyear, accounting for 60percent of the global market.

Being a Scot, John Boyd Dunlop would be spinning in his grave if he could see the prices of new tyres today. A car owner on a tight budget therefore has to choose replacement tyres wisely and then get maximum service out of them.

The Road Traffic Act states that tyres should have across their breadth and around their entire circumference a tread of at least 1mm in depth. No bald patches, no smooth shoulders. Half a match-stick is 1mm. Less than that means you are contravening the law. In fact, by the time you are down to 1mm, you are already courting disaster. The aim of this legislation is to protect the motoring public and pedestrians from the dangers of near-smooth tyres. When you have to stop suddenly from 50kmh in wet weather, say for a child running across the road, your stopping distance is nearly doubled if your tread depth is 1mm as opposed to 10mm. Tyre manufacturers provide "wear bars" inside the grooves of the tread. When the tread has worn down to the wear bars, it's really time to replace that tyre, and the act stipulates that a tyre may not be used on a public road if "the tread is level with the tread depth indicator".

Once you have made peace with the idea that you will have to fork out hard-earned money for replacement tyres, the next decision is - what kind of replacements? Looking at new tyres - always the first prize if you can afford it - we see a variety of makes and models on the market. Broadly, the field can be divided into three categories: specialist tyres, high performance, or off-road, or taxi tyres; basic tyres for the faithful family car; and budget tyres for people who cannot resist a bargain, though experience has shown that if something sounds too good to be true, it usually is.

In the middle group the Big Four in South Africa - Continental, Dunlop, Firestone and Goodyear - are all strong contenders. They are closely matched in quality and price. Tyre engineers always face the trade-off between good roadholding but faster wear, on the one hand, and long life but less grip, on the other.

Any tyre in the middle group will be a compromise between these conflicting requirements and it's only the mix that differs.

The steel-belted radial tyre has 100percent of the original and replacement market. It consists of an inner layer of impermeable rubber to prevent air from escaping under pressure, a casing, a tread underpinned by steel belts, and sidewalls covering the outside of the casing.

Sidewalls display interesting information about the tyre. A date code tells you when the tyre was manufactured. Since 2000 it consists of four digits, the first two giving the week and the last two the year. The tyre size, aspect ratio, load index, and speed rating will be shown, usually grouped together. The traction rating, AA is best, C is worst, tells you how much grip the tyre will have on a wet road. The temperature rating shows how well the tyre can cope with heat. A is best, C means it was meant for a cold country.

Next week we look at how best to care for these precious pieces of black rubber that make such a big dent in our budget.