Tyres: legal tread depth, pressures, rotation and when to replace

Tyres are the only part of the car that touches the road, so they have a big effect on braking, steering and grip, especially in the wet. The good news is that a few simple checks go a long way.

Legal tread depth

In Australia, the legal minimum tread depth for car tyres is 1.5 mm. Most tyres have small raised bars moulded into the bottom of the main grooves, called tread wear indicators. When the tread has worn down level with those bars anywhere on the tyre, it’s at or near the legal limit and needs replacing.

The legal minimum isn’t the same as ideal. Grip in the wet drops noticeably as tread wears, well before the legal limit, so it’s worth replacing tyres before they get that far.

Driving on tyres that are below the legal limit, or that have damage such as exposed cords or bulges, can lead to a defect notice or a fine. The car also won’t pass a safety inspection.

Tyre pressures

The right pressure helps the car brake and corner properly, helps the tyres wear evenly and can help fuel use.

• Find the recommended pressures on the tyre placard, usually on the driver’s door frame or inside the fuel flap, or in your owner’s manual. Don’t use the maximum pressure printed on the tyre’s sidewall.
• Check pressures when the tyres are cold, for example before a long drive or first thing in the day.
• Check them about once a month, and before long trips or carrying a heavy load. Some placards list higher pressures for a fully loaded car.
• Don’t forget the spare, if your car has one.

If one tyre keeps losing pressure, it may have a slow puncture or a leaking valve. Have it checked rather than topping it up again and again.

Rotation

Front and rear tyres usually wear at different rates. Rotating them, which means moving them to different positions on the car, helps them wear more evenly and last longer. Your logbook or owner’s manual gives the recommended interval for your car, and it’s often done as part of a regular service. Some cars have different-sized front and rear tyres, or directional tyres, which limits how they can be rotated.

Wheel alignment

If the tyres are wearing unevenly, for example on the inside or outside edge, or the car pulls to one side or the steering wheel sits crooked when you’re driving straight, the wheel alignment may be out. Hitting a kerb or a pothole can knock it out. Uneven wear is a good reason to have it checked.

When to replace a tyre

Replace a tyre if:

• the tread is at or near the tread wear indicators
• there are cuts, bulges, cracks or exposed cords
• it has been damaged by being driven on while flat
• it has a puncture that can’t be safely repaired, such as one in the sidewall

Tyres also age, even if they have plenty of tread left. The rubber hardens and can crack over time, particularly in strong sun. The date a tyre was made is printed on the sidewall as part of a code: the last four digits show the week and year of manufacture. For example, 2320 means the 23rd week of 2020. If your tyres are getting old, ask for them to be checked, and follow any age guidance in your owner’s manual.

It’s generally best to replace tyres in pairs on the same axle, with matching size and type, so the car handles evenly.

Get them checked

If you’re not sure whether your tyres are due, or you’ve noticed uneven wear, vibration or pulling, call Ridgeline Auto on 02 9000 0000 and we’ll take a look.