When to Replace Your Tyres (And Why Waiting Costs You More)

Many NZ drivers run their tyres until they're nearly bald, thinking they're saving money. In reality, worn tyres cost you more — in fuel, in safety, and potentially in WoF failures. Here's how to know when it's time to replace, and why the sweet spot is earlier than you think.

The Legal Minimum (And Why It's Not Enough)

In New Zealand, the legal minimum tread depth is 1.5mm across the main tread grooves. If your tyres are below this, you'll fail your WoF and can be fined $150 per tyre by police.

But 1.5mm is a safety floor, not a recommendation. At 1.5mm, your tyres have lost the vast majority of their wet-weather grip. Braking distances on wet roads can be double or triple what they are with new tyres.

"We recommend replacing tyres at 3mm. Below that, wet grip deteriorates rapidly and the risk of aquaplaning increases significantly." — NZTA (Waka Kotahi)

How to Check Your Tread Depth

The $2 Coin Test

Insert a NZ $2 coin into the main tread groove with the "2" facing down. If you can see the entire "2", your tread is below 3mm and it's time to start shopping for new tyres. If the tread covers part of the "2", you still have life left.

Tread Wear Indicators

All modern tyres have tread wear indicators (TWI) — small raised bars moulded into the bottom of the main grooves. When the tread surface is level with these bars, you've reached 1.5mm and the tyre is legally worn out. Look for small triangles or "TWI" markings on the sidewall to locate them.

Tread Depth Gauge

For precision, use a tread depth gauge ($5–$10 from any auto store). Measure in at least three points across the tread width and at multiple locations around the circumference. The lowest reading is your effective tread depth.

Uneven Wear: What It Tells You

Uneven wear means you're not getting the full life out of your tyres. A wheel alignment ($80–$120) can extend tyre life by 20–30% and improve fuel economy by 1–2%. It's one of the best-value maintenance items you can do.

Tyre Age: The Hidden Factor

Even if your tread looks fine, tyres degrade with age. The rubber compounds oxidise, harden, and lose flexibility over time — regardless of how much you drive. Most manufacturers recommend replacing tyres after 6 years from the date of manufacture, and no tyre should be used after 10 years.

How to Read the DOT Date Code

Look for a four-digit number on the sidewall, usually preceded by "DOT". The last four digits indicate the week and year of manufacture. For example, 2523 means the tyre was made in week 25 of 2023.

This is especially important when buying tyres — always check the DOT code. A "new" tyre that's been sitting in a warehouse for 3 years has already used half its shelf life.

The Fuel Cost of Worn Tyres

Here's the counterintuitive part: new tyres actually have higher rolling resistance than partially worn ones. More rubber deforming means more energy lost. As tyres wear from 8mm to 3mm, rolling resistance decreases and fuel economy slightly improves.

But below 3mm, you lose so much wet grip that the safety trade-off isn't worth the marginal fuel saving. And once you replace with modern fuel-efficient tyres, the new set will have lower rolling resistance than your old worn tyres ever did — thanks to advances in compound technology.

The Real Cost of Waiting

Drivers who run tyres to 1.5mm instead of replacing at 3mm save roughly $0 on fuel (the rolling resistance difference is negligible at that point) but:

• Increase wet braking distance by 30% — at 80km/h, that's an extra 12 metres. Enough to be the difference between stopping and a collision.
• Risk aquaplaning — worn tyres can't channel water away from the contact patch. At 80km/h on a wet road, tyres below 2mm are effectively floating.
• Fail their WoF — a failed WoF means your car is unregistered until fixed, plus the cost of a recheck ($50+).
• Risk a fine — $150 per bald tyre if stopped by police, plus demerit points.

When to Replace: The Summary