Tyre Tread Depth

Tyre tread depth is the fundamental physical factor determining tyre performance and serves as the most direct indicator of whether a tyre is safe for the road. Beyond affecting wet water dispersion performance, it is directly linked to dry grip and braking distances.

Key Values and Limits

Within the automotive industry, tyre tread depth is typically measured in millimetres (mm).

New Tyre Depth: Most passenger car tyres feature an initial tread depth between 7mm and 9mm. 

Legal Replacement Limit (1.6mm): Across nearly all traffic regulations globally, 1.6mm is the statutory minimum depth limit. Once worn down to this level, the vehicle will fail roadworthiness inspections and faces an extremely high risk of aquaplaning in wet conditions.

Recommended Replacement Limit (3mm - 4mm): Due to a noticeable degradation in wet braking performance, many tyre manufacturers and safety bodies recommend replacing tyres once tread depth wears down to between 3mm and 4mm, particularly in high-rainfall regions.

The Physics of Water Dispersal

The primary function of tyre tread is water evacuation. When travelling at higher speeds on wet roads, the tread grooves act as temporary channels to funnel and displace standing water away from the contact patch, ensuring direct contact between the tyre rubber and the tarmac.

As Tread Depth Decreases: The volume of the drainage channels is reduced. When standing water exceeds the capacity of the grooves, water cannot be dispersed fast enough, causing the tyre to "float" on a film of water. At this point, the vehicle completely loses steering and braking control—the fundamental cause of aquaplaning.

How to Measure Tyre Tread Depth?

Tread Wear Indicators (TWI): Every tyre features small raised rubber bars within the main grooves. Once the tread wears down flush with these markings, it indicates that the tread depth has reached the critical 1.6mm threshold. 

Tread Depth Gauge: A dedicated tyre depth gauge provides higher measurement accuracy. 

Coin Test (Quick Check): In the absence of proper tools, a coin can be inserted vertically into the groove (using the outer rim or design on the reverse side as a gauge). If the outer rim is fully concealed by the tread, the depth remains acceptable; if the tread has worn down to expose the outer details on the coin's reverse, it indicates insufficient tread depth.