Manufacturers recommendation is to always fit new tyres to the rear of a vehicle. Why?
With new tyres fitted in front, the behavior of the car will change, because the front / rear balance will be reversed. The driver, used to a car with less grip at the front, may therefore be taken unawares. On a slippery road, the rear will tend to lose traction before the front of the vehicle. The driver will have little or no chance of controlling the rear, and will be tempted to accelerate further, which will amplify the spin effect. Only an experienced driver will be able to recover from this dangerous situation.
Every tyre on a motor car must have at least 1.6mm of tread depth across the central 3/4 of the tyre tread and visble tread on the remainder of the tyre. The picture opposite shows a tread wear indicator. This is a small raised portion of rubber in the tread area adjacent to the arrow on the shoulder. When this raised portion is level with the tread grooves then the tyre requires replacing
Manufacturers recommend replacing tyres when the tread depth gets down to 3mm. Below this depth and the treads ability to remove water is greatly reduced.
As can be seen from the graph, wet road grip falls away dramatically as a tyre wears down. In fact there is a 60% reduction in grip between 3 and 1.6 mm tread depth.
The possibility of the vehicle aquaplaning is also increased by the same amount between these two figures.
Correct inflation is very important in that if the tyre is under inflated then the centre of the tread tends to lift thereby reducing the contavct area between tyre and road. If the pressure is too high then the edges of the tread lift and contact area is again lost. There is also an adverse effect on the wear rate of the tyre.
