The inflation pressures of all tyres, including the spare, should be checked at least once a week and adjusted if necessary. The recommended pressures are given in 'Service Data' at the end of this section. Check the pressures when the tyres are cold as tyre pressures may increase by as much as 6 psi (0.4 kg/cm') when hot.
Incorrect inflation pressures will cause abnormal tyre wear and may result in premature failure. There is an average loss of 13% tread mileage for every 1 Psi reduction in inflation pressures below the recommended figure.
After checking the pressures, always ensure that the dust cap is refitted on each valve as this excludes any dirt and provides an additional seal for the valve.
Tyre Condition The condition of the tyres should be checked periodically as a safety measure. Measure the depth of tread of each tyre, preferably with a proper tread depth gauge. Tyres should be replaced, at the latest, when the tread depth has reached 1 mm all round and on full tread width, as this is the absolute limit for safe driving. However, it is higly recommended that tyres be replaced before this as they will be more susceptible to punctures and have greatly reduced grip, especially in the wet.
Check the tyre casing visually for cuts in the casing fabric, exposure of ply or cords, or the presence of lumps or bulges. If any of these conditions are present, the tyre should be discarded. Abnormal tyre wear may be caused by improper inflation pressures, wheel imbalance, suspension misalignment, or mechanical irregularities. When rapid or uneven tyre wear becomes apparent, the cause should be established and dealt with.
Fins or feathers on the tread surface are an indication of severe wheel misalignment. This condition takes the form of a sharp 'fin' on the edge of each pattern rib, and the position of this indicates the direction of misalignment. Fins on the outboard edges are caused by excessive toe-out, whereas fins on the inboard edges of the pattern ribs are caused by excessive toe-in.
Firming on the near-side front tyre only may be due to severe road camber conditions and cannot be eliminated by mechanical adjustment. In this event, frequent interchanging of the affected wheel to even out tyre wear is the only solution.
Some mechanical defects which should be a cause of abnormal tread wear are: loose or worn wheel bearings, uneven braking due to dragging brakes, seized wheel cylinders or distorted brake discs, excessive looseness or damage in the suspension, loose steering connections, bent steering arms, or defective shock absorbers.
Tyre Replacement Either radial or cross-ply tyres may be fitted as standard equipment and replacements should be of the same type. It is also recommended that only the specified size and type is fitted. Only tyres of the same type and tread pattern should be used on one vehicle. Do NOT mix crossply and radial ply tyres, or textile and steel cord tyres.