Tyre tread isn't just about passing a roadworthy inspection—it's a critical safety feature that determines how effectively your vehicle can grip the road, disperse water, and maintain control in challenging conditions. Understanding how to accurately measure tread depth and interpret wear patterns empowers you to make informed decisions about tyre replacement, potentially preventing dangerous situations before they occur.
Why Tread Depth Matters
The grooves in your tyre tread serve a vital purpose: channelling water away from the contact patch between rubber and road. When driving on wet surfaces, a well-treaded tyre can disperse up to 30 litres of water per second at highway speeds. As tread wears down, this water evacuation capacity diminishes dramatically.
At the legal minimum of 1.5mm, a tyre's wet weather stopping distance can increase by up to 40% compared to a new tyre with 8mm of tread. This means that in an emergency braking situation at 100 km/h, a worn tyre might require an additional 15-20 metres to stop—the difference between a close call and a serious collision.
Beyond water dispersal, tread depth affects traction on loose surfaces, braking performance on all surfaces, and the tyre's ability to maintain shape during cornering. Adequate tread is fundamental to vehicle safety.
Method 1: Tread Wear Indicators
Every modern tyre manufactured for the Australian market includes built-in tread wear indicators (TWIs). These are small raised bars, typically 1.6mm high, located in the main circumferential grooves of the tread pattern. You can find them by looking for small triangular markers or the letters "TWI" moulded into the sidewall—these point to the indicator locations.
When your tread wears down to the level of these indicators, the tyre has reached the legal minimum depth and must be replaced. The beauty of TWIs is their simplicity—they require no tools and provide an immediate visual reference.
Look around the tyre sidewall for small triangle symbols or "TWI" markings. These indicate where the wear indicators are located within the main tread grooves. Check at least three positions around the tyre's circumference for accurate assessment.
Method 2: The Tread Depth Gauge
For precise measurements, a tread depth gauge is an inexpensive and invaluable tool. Available from automotive stores for under $15, these simple devices provide exact millimetre readings. Digital versions offer even greater precision, though mechanical gauges are perfectly adequate for general use.
To use a tread depth gauge:
- Insert the probe into the deepest part of a main groove (not the small sipes or secondary channels)
- Push the gauge body down until it rests firmly on the tread surface
- Read the measurement from the scale
- Repeat in multiple locations across the tyre width and at different points around the circumference
- Record the lowest reading—this is your effective tread depth
Take measurements in at least six locations per tyre: inner edge, centre, and outer edge, each at two points 180 degrees apart around the tyre. This comprehensive approach reveals both overall wear and any concerning patterns.
Method 3: The Coin Test
If you don't have a gauge handy, a twenty-cent coin can provide a rough estimate. Insert the coin into a main groove with the platypus facing down. On an Australian twenty-cent piece, the distance from the edge to the bill is approximately 3mm. If you can see the entire bill, your tread is below the recommended replacement point.
While less precise than a gauge, the coin test is useful for quick roadside checks or when inspecting a potential used vehicle purchase.
- New tyre: 7-8mm typical
- Good condition: 4-6mm
- Consider replacement: 3mm
- Legal minimum (Australia): 1.5mm
- Dangerous: Below 1.5mm
Reading Wear Patterns
Tread wear isn't just about depth—the pattern of wear reveals important information about your vehicle and driving habits. Learning to read these patterns helps you address underlying issues and extend tyre life.
Centre Wear
When the middle of the tread wears faster than the edges, your tyres are likely over-inflated. Excessive pressure causes the centre of the tread to bulge outward, concentrating wear in that area. Check your inflation pressure and adjust to the manufacturer's specification.
Edge Wear
Wear concentrated on both outer edges indicates under-inflation. Without sufficient pressure, the tyre sidewall flexes inward, causing the edges to bear disproportionate load. Beyond accelerated wear, this condition generates excessive heat and increases the risk of tyre failure.
One-Sided Wear
If one edge of your tyre is wearing significantly faster than the other, your wheel alignment is likely off. This is particularly common on the front tyres and often develops gradually after hitting potholes or kerbs. A professional alignment check should resolve the issue.
Cupping or Scalloped Wear
Wavy, uneven wear around the tyre circumference suggests suspension problems—worn shock absorbers, struts, or bushings allow the wheel to bounce rather than maintain consistent road contact. This pattern indicates a mechanical issue requiring attention beyond tyre replacement.
Unusual wear patterns don't just waste tyres—they indicate mechanical issues that affect safety and vehicle control. If you observe any abnormal wear, have your vehicle inspected by a qualified mechanic before fitting new tyres. Replacing tyres without addressing the underlying cause will simply result in the same premature wear on the new set.
When to Check Tread Depth
Develop a routine for regular tread depth assessment. Monthly checks are ideal for most drivers, but increase frequency if you cover high kilometres or drive in challenging conditions. Always check before long journeys, as extended highway driving generates heat that can accelerate wear on already marginal tyres.
Additionally, check after your vehicle has been parked for extended periods. Tyres can develop flat spots or degradation that affects their contact patch, even without accumulating kilometres.
Replacing at the Right Time
While 1.5mm is the legal minimum, waiting until this point significantly compromises safety. Most safety experts recommend replacement at 3mm, particularly if you frequently drive in wet conditions or on highways. At 3mm, you still have adequate tread for dry conditions but are approaching the threshold where wet weather performance deteriorates noticeably.
Consider your driving patterns when deciding. If you primarily drive in dry conditions on familiar urban roads at moderate speeds, running to 2mm might be acceptable. But if your routine includes highway driving, regular rain exposure, or you carry passengers frequently, earlier replacement provides an important safety margin.
Remember that tread depth is measured at the shallowest point. If any portion of your tyre's tread pattern has reached minimum depth, the entire tyre needs replacement, even if other areas show more rubber remaining.
Building Good Habits
Making tread depth checks part of your regular vehicle maintenance ensures you're never caught off-guard by worn tyres. Keep a simple gauge in your glovebox or garage, and take a minute during each fuel stop to glance at your tyres. This small investment of attention pays dividends in safety, and catching wear early lets you budget and plan for replacement rather than facing an unexpected expense.