Quick answer: Replace a tyre when it hits the 1.5mm legal tread limit — but also when it's around 10 years old regardless of tread, when the sidewall is cracked or bulging, when it wears unevenly, when it vibrates or is noisy, or when it's had repeated or irreparable punctures. Tread depth is just one of at least six reasons a tyre reaches the end of its life.

Most drivers only ever check one thing: how much tread is left. Tread matters — in Australia the legal minimum tread depth is 1.5mm across all principal grooves, and safety bodies suggest replacing at around 3mm for confident wet grip. But a tyre can be dangerous, or illegal, with plenty of tread left. Here are the six signs that matter, and why each one ends a tyre's life.

1. The tread has reached the wear limit

Start with the obvious one. Once any principal groove wears to 1.5mm the tyre is illegal and its wet braking and aquaplaning resistance have fallen off a cliff. Every tyre has built-in tread wear indicators — small raised bars in the grooves that sit flush with the tread at 1.5mm. When the tread is level with those bars, it's done. Don't wait for it; grip in the wet is already poor well before the legal limit.

2. The tyre is old, whatever the tread looks like

Rubber ages whether you drive on it or not. Industry and manufacturer guidance is to have tyres inspected yearly once they pass about five years, and to replace them at around 10 years regardless of how much tread remains — there is no legal expiry date in Australia, but the rubber's strength and grip fade with time. Read the DOT date code to find the age, then see should you replace tyres over five years old and do tyres expire. A five-year-old tyre with full tread on a car that barely leaves the garage is a classic trap.

3. The sidewall is cracked or perished

Fine surface crazing is often cosmetic, but deep, wide cracks — or any crack reaching into the tyre's cords — mean the casing is failing and the tyre must be replaced. Australia's sun makes this worse; see UV damage and dry rot and our detailed guide to sidewall cracking: when it's cosmetic and when it's dangerous. Dry rot is not repairable — a cracked, perished sidewall is a replacement, not a patch.

4. There's a bulge or bubble in the sidewall

A bubble on the sidewall means the internal plies have been damaged — usually from a kerb or pothole strike — and air has pushed into the rubber. It is a blowout waiting to happen and it cannot be repaired. Replace it straight away and don't drive far on it. If you've recently clouted a kerb or hit a pothole, inspect the sidewalls closely — bulges can appear days later.

5. The wear is uneven

A tyre that's worn out on one edge, cupped, or feathered has usually been telling you about a deeper problem — alignment, worn suspension, or wrong pressures. The tyre itself may need replacing early, and the underlying fault must be fixed or the new tyre wears out the same way. Learn to read tyre wear patterns like a mechanic so you catch the cause, not just the symptom.

6. It vibrates, shakes or is getting noisier

Persistent vibration at highway speed that a rebalance won't cure, or road noise that keeps getting worse, can point to internal tyre damage, a thrown belt, or irreversible cupping — all signs a tyre is finished. If a fresh balance and a wheel inspection don't fix it, the tyre is often the answer.

Replace or monitor? A quick reference

Sign	Replace now?
Tread at 1.5mm (wear bars flush)	Yes — illegal and unsafe
Around 10 years old	Yes, regardless of tread
Deep sidewall cracks / dry rot	Yes — not repairable
Sidewall bulge or bubble	Yes, immediately
Uneven wear	Often — and fix the cause
Vibration/noise a balance won't cure	Likely — inspect first
Repeated or irreparable punctures	Yes if outside the repairable zone

That last row matters: a puncture is only repairable in the central tread, and only within limits. See where on a tyre can legally be repaired in Australia — a shoulder or sidewall puncture means replacement, not repair.

Frequently asked questions

Do I have to replace all four tyres at once?

Not always. On many cars you can replace in pairs (fit the newest tyres to the rear for stability). AWD vehicles are stricter — they often need closely matched tread across all four to protect the driveline. When you do replace just one or two, it's largely a cost decision.

Is 3mm really the time to replace, not 1.5mm?

1.5mm is the legal limit; 3mm is the safety recommendation. Wet braking distances and aquaplaning resistance drop sharply below about 3mm, so many drivers replace there for a margin of safety, especially before a wet season or a long trip.

How do I check the age of my tyres?

Find the DOT code on the sidewall; the last four digits are the week and year of manufacture (for example 2224 = the 22nd week of 2024). That's the true age, which can be very different from when the tyre was fitted.

Can worn tyres fail a roadworthy or safety inspection?

Yes. Tyres below 1.5mm, or with cracks, bulges, exposed cords or unsafe repairs, will fail a state inspection anywhere in Australia and can attract a defect notice and fine.

Bottom line

Tread depth is the floor, not the whole test. A tyre can be legal on tread and still be finished — too old, cracked, bulging, wearing unevenly or shaking. Check all six signs, and when in doubt about age or damage, get a qualified fitter to look. To set expectations for how long a set should last in the first place, see how long should tyres last.

General guidance for Australia, 2026. Legal tread minimum is 1.5mm in all principal grooves; there is no legal tyre expiry date. Confirm inspection rules with your state road authority and have any tyre you're unsure about checked by a qualified tyre fitter.