What tread depth should your tyres have?
A tyre's tread is made up of grooves that clear water from under the tyre on wet roads. As the tread wears, these grooves get smaller and the tyre starts to risk "skating" on water (aquaplaning) sooner. That's why tread depth is the most basic value to look at when deciding when to replace a tyre.
Tread depth on a new tyre
A new passenger car summer tyre usually has a tread depth of 7–9 mm. On winter tyres this value is slightly higher, usually 8–10 mm. Even deeper treads are found on SUV and light commercial tyres.
Legal minimum
In Turkey and most of Europe, the legal minimum tread depth for passenger car tyres is 1.6 mm. Driving on tyres below this value can lead to a penalty and is a reason for failing the vehicle inspection.
A winter tyre needs deeper tread to do its job in snow. For vehicles covered by the winter tyre rules, a lower limit of 4 mm is generally required.
The legal limit is not a safe limit
1.6 mm is the point at which the tyre can no longer be used. Braking performance on wet surfaces starts to drop long before this. Tyre manufacturers' tests show that wet braking distances lengthen quickly once tread depth falls below 3 mm.
The general recommendation is:
- Summer tyres: Plan a replacement when they reach 3 mm.
- Winter tyres: When they reach 4 mm, consider new ones for the next winter.
Tread wear indicator (TWI)
At the bottom of a tyre's main grooves there are small raised bars about 1.6 mm high. These are called tread wear indicators. Their positions are marked on the sidewall with a triangle or the letters "TWI".
If the tread has worn down to the level of these bars, the tyre has reached the legal limit and must be replaced immediately. Some tyres also have numbers moulded into the tread surface or separate indicators for winter use.
How to measure tread depth
Tread depth gauge
This is the most accurate method. Gauges are cheap and available from tyre shops and motor accessory stores. Press the tip straight down so that it touches the bottom of a main groove and read the value.
Coin
As a practical method, you can also use a coin. Measure the distance between the edge of the coin and the design on it once with a ruler, then place the coin in the groove and see whether the design is visible. This method gives a rough idea; use a depth gauge for an exact measurement.
Where to measure
Measuring at a single point on the tyre can be misleading. On each tyre, take measurements:
- From the inner, centre and outer grooves,
- At several different points around the tyre.
The lowest value shows the tyre's true condition.
What does uneven wear tell you?
| Wear pattern | Possible cause |
|---|---|
| Both edges excessively worn | Low pressure |
| Centre excessively worn | High pressure |
| One edge excessively worn | Wheel alignment (camber, toe) out |
| Wavy, like saw teeth | Shock absorber or balancing problem |
| Localised flat spot | Wheel locking under hard braking |
If you see uneven wear, fix the cause before replacing the tyre. Otherwise the new tyre will wear in the same way.
Front and rear tyres
On front-wheel-drive cars the front tyres usually wear faster. Swapping tyres between front and rear at regular intervals (rotation) evens out the wear. If you are buying two new tyres, it is generally recommended to fit the new tyres at the rear to keep control of the car.