Knowing when to replace tyres isn’t guesswork. Tyres tell you they’re done — in eight pretty specific ways. The trouble is most drivers either don’t notice the warning signs, or notice and assume it can wait until the next service. Both habits cost a lot when something lets go on the freeway.
This is the same checklist we use on every free tyre check at Tyre Doctors Knoxfield. Any one of these by itself is worth booking an inspection. Two or more and you’re almost certainly buying tyres this week.
1. Tread depth is at or below 3 mm
The legal minimum in Australia is 1.5 mm. The safe minimum is closer to 3 mm — that’s the point at which wet-weather grip drops sharply and stopping distances in the rain start to grow significantly.
The easy at-home test: stick a 20-cent coin into the deepest part of the tread. If you can see the platypus’s bill clearly above the tread, you’re under 3 mm. Replace soon. If the platypus is fully visible from edge to edge, you’re at or near the legal limit. Replace now.
Most tyres also have small raised bars across the bottom of the tread grooves — the “tread wear indicators”. When the tread is worn flush with these bars, the tyre is at 1.6 mm and legally done.
2. Cracks in the sidewall or between tread blocks
Rubber dries out and cracks over time, especially when the car is parked outside in Melbourne summers. Look for:
- Fine spider-web cracks running across the sidewall (called “crazing”).
- Deeper cracks at the base of tread blocks.
- Cracks visible when you push your thumb into the tyre.
Light surface cracking on tyres under 5 years old is usually cosmetic. Deeper cracks, or any cracking on tyres over 6 years old, means the rubber compound is failing. Replace.
3. Bulges, blisters or splits on the sidewall
This is the one that scares us most. A bulge or blister on the sidewall means the internal structure of the tyre — the steel belts or fabric plies that hold the shape — has failed. The rubber is the only thing holding the air in.
A sidewall bulge can let go without warning at any speed, but it’s most dangerous at freeway speeds when the tyre is hot. If you see a bulge bigger than a 5-cent coin, don’t drive on it. Get the car to a workshop on the spare or have it towed.
Common causes: hitting a kerb at speed, hitting a deep pothole, or repeatedly driving on the tyre while under-inflated.
4. Uneven wear across the tread
Run your hand across the tread of each tyre, from inside shoulder to outside. It should feel smooth and even. Common warning patterns:
- Inside shoulder bald, outside fine: Negative camber out of spec, or worn suspension. Needs a wheel alignment check.
- Outside shoulder bald, inside fine: Positive camber out, or chronic under-inflation.
- Centre band bald, shoulders fine: Chronic over-inflation, or constant freeway driving on warm tyres.
- Both shoulders bald, centre fine: Chronic under-inflation.
- Cupping or scalloping (raised and worn patches in a pattern around the tyre): Worn shock absorbers, or out-of-balance wheels.
- Feathering (raised edges on one side of each tread block): Toe alignment out of spec.
Uneven wear means the tyre is finished AND the underlying cause needs fixing. Otherwise the new tyre will wear the same way.
5. Steering vibration above 80 km/h
If the steering wheel shakes at freeway speeds but is fine in the suburbs, it’s almost always wheel balance. Tyre weights can fall off, especially after a kerb hit, and the tyre starts spinning with a heavy spot.
Sometimes vibration means the tyre itself is failing — internal belt separation, in which case the tyre is unrepairable and needs replacement. A workshop can tell the difference quickly by spinning the wheel on a balancer.
Either way, don’t ignore it. Vibration accelerates suspension wear too.
6. The car pulls to one side on a flat, straight road
Pick a flat section of road — somewhere on the Eastern, Monash or EastLink works — and lighten your grip on the steering wheel. The car should track dead straight. If it drifts toward the kerb or the centre line, something’s wrong.
Common causes:
- Tyre pressures uneven left vs right (easy fix).
- One tyre is worn or damaged differently to its pair (replace).
- Wheel alignment is out (book an alignment).
- A brake is dragging on one side (book a brake check).
If a pressure top-up doesn’t fix it, you need a workshop to diagnose.
7. Slow leaks needing weekly top-ups
A tyre that loses 1–2 PSI per month is normal. A tyre that loses 5–10 PSI per week has a leak — usually a nail, a damaged valve, or a corroded wheel bead.
Some leaks are repairable (a clean nail puncture in the central tread area, away from the sidewall). Some aren’t (anything in or near the sidewall, large punctures, second punctures in the same tyre). Our puncture repair page covers what’s safely repairable — a workshop needs to inspect to confirm.
Driving on a slowly leaking tyre that drops below 25 PSI ruins the sidewall internally, even when you pump it back up. The damage is usually invisible from the outside.
8. Tyres are over 6 years old (regardless of tread)
Find the four-digit DOT date code on the sidewall — for example, 2622 means the 26th week of 2022. If the tyre is past 6 years from that date, the rubber compound is starting to harden, and grip in the wet drops noticeably even with full tread depth.
Most tyre manufacturers recommend replacement at 6 years, and almost no one will warrant a tyre past 10 years from manufacture. If you bought the car used, check the DOT date — sometimes “nearly new” tyres on a used car are actually 8 years old and never been driven.
What to do next
If any of the above is true for your car:
- Stop driving immediately if you see cord/belts, a bulge, or a split sidewall.
- Book a free tyre check this week if you see uneven wear, cracking, slow leaks, vibration or pulling.
- Plan a tyre replacement in the next month if your tread is at 3 mm or your tyres are 5+ years old.
A free tyre check at Tyre Doctors takes 15 minutes, costs nothing, and includes a written report showing tread depth in mm, age, and condition of each tyre. We’ll tell you straight: safe, monitor, or replace now.
Book a free tyre check today
Drop in to 5/1644 Ferntree Gully Road, Knoxfield, or call 03 9763 0100. Open Monday to Friday 8–5, Saturday 10–3. If you do need new tyres, we stock all major brands and most fitments are same-day.