Tyres

Markings

It is important that tyres are suitable in all respects for the vehicle to which they are fitted. Tyres carry internationally standardised identification markings that should be treated in the same way as a part number to ensure the correct choice of tyre.

Dimensions

All dimensions stated refer to a new inflated tyre on a measuring rim, which is the control rim used as a reference to determine dimensional data for a given tyre.

A. Section width Total inflated width of the tyre at the widest point, excluding kerbing ribs and lettering. When a tyre is fitted to a wider or narrower rim than the measuring rim, the section width will change by approximately 5mm for each 0.5" change in rim width.

B. Section height The height from tyre bead to crown

C. Overall diameter The diameter of a new tyre fitted and inflated on a rim in an unloaded condition, i.e., off the vehicle

D. Static loaded radius The distance between wheel centre and road surface with the tyre laden

Rolling circumference: The distance an inflated and loaded tyre will roll in one revolution at 100kph

Revs/Km: The number of revolutions per kilometre of a tyre in a loaded condition at 100kph

Size designation

Together, these indicate the tyre dimensions, structure, load capacity and speed rating.  A typical example found on the sidewall of a tyre is 185/60R14 82H.

• P - Passenger car and T indicates a temporary spare tyre
• 185 - The appropriate tyre section width in mm when fitted to its measuring rim
• 60 - The tyre aspect ratio. In this example, the tyre section height is 60% of its section width.
• R - indicates the tyre is of 'radial' construction
• 14 - the diameter of the rim to which the tyre must be fitted - 14"
• Service Description - the combination of the load index and speed symbol
• Load Index (LI) - a numerical code indicating the maximum load a tyre can carry at the speed indicated by its speed symbol
• Speed Symbol - an alphabetical code indicating the speed at which the tyre can carry a load corresponding to its load index

Taking care of your tyres

Keep your tyres inflated correctly. Inflate tyres to your manufacturers recommendations and recheck once a month. The recommended tyre pressure is usually recorded on the pillar of the driver’s door. Under inflated tyres result in increased road friction, meaning that the tyres will wear faster and require move fuel to run the car.

All four tyres should be of the same tread pattern.  Ideally, the spare tyre should be identical. If fitting new tyres, put them on the same axle - preferably the rear. Regularly check the tread depth and tyre pressures (including the spare).

Match your tyres

Driving with mismatched tyres is dangerous. Among other warrant of fitness requirements, tyres on the same axle must be of the same tread pattern. However, there is no legal requirement for tyres on all axles to be the same tread, even though NZTA agrees that fitting four identical tyres (preferably summer) provides the best grip.

Snow tyres are made with a softer rubber compound and deeper treads, designed to maintain grip in ice and snow. They can be distinguished by their deep square-patterned tread blocks with numerous fine blades. Newer snow tyres are also marked with a 'mountain snowflake' symbol on the sidewall. They might be found on used cars imported from colder parts of Japan. In Japan, such tyres must have at least 50% tread depth to be legal, yet many below this will still comply with NZ's minimum tread depth of 1.5mm and thus can be used here.

NZTA says the majority of Kiwi motorists have no need to use snow tyres - those who do drive in ice or snow should have them fitted to all four wheels and change them at the end of winter.

However, snow tyres must not be confused with all-season 'mud and snow' tyres (often marked with the letters 'M+S' on the sidewall), which are designed to perform safely in a wide range of summer and winter conditions and are commonly fitted to 4WD vehicles.

Check the tread pattern on all four wheels, and if any appear to be specialist snow tyres, arrange for these to be replaced with ordinary summer tyres. Have your car inspected immediately if you've ever experienced any loss of control, particularly if it's a Japanese import.

Snow tyres

New rules concerning the use of winter (or snow) tyres came into effect on 1 April 2010. The regulations prohibit mixing winter tyres with normal tyres on the same vehicle.

As part of the WoF check, a vehicle will now fail if it has mixed tyres. A new minimum winter tyre tread depth of 4mm also applies, compared to 1.5mm for standard tyres.

Snow tyres are made with a softer rubber compound and deeper treads specifically designed to maintain grip in ice and snow. They can be distinguished by their deep square-patterned tread blocks with numerous fine blades. Snow tyres may also be marked with a snowflake and mountain symbol on the sidewall or the word 'studless'. They tend to be found on used cars imported from colder parts of Japan.

The rule change follows several fatal accidents involving vehicles fitted with a mix of normal and winter tyres, and subsequent testing which showed that such combinations can result in different levels of grip on the front and rear axles, which can lead to a loss of control, particularly under emergency braking or in the wet.

The AA recommends all four tyres should be of the same tread pattern as well as construction. Likewise, the spare tyre should be identical, but if not then you should only use it as a short-term emergency until the regular tyre is repaired or replaced. If fitting new tyres, put them on the same axle - preferably the rear.

If in doubt, visit your local tyre outlet for advice or contact the AA Motoring Helpline on 0800 500 333.