Tire tread depth: Why you should replace early for better safety

Adequate tread depth in tires is extremely important, especially when driving on wet surfaces and, in the case of winter tires, also on snow. Manufacturers place a TWI indicator that shows the tire's wear level. Recently, a new solution has been the use of numbers on the tread.

As we see in the photos, Nokian first introduced these markings. Continental and Goodyear also use them, so they can be found on tires from various manufacturers. The mysterious numbers indicate the tread groove depth in inches. Disappearing numbers indicate worn tread.

The smallest visible number is 5/32. Such a value suggests that a tread thickness of 5/32 inches no longer provides optimal performance for either summer or winter tires. When only the "5" remains, the tire is still usable, but replacement should be considered. When this value also disappears, immediate replacement of the tire is necessary.

Tread grooves have a primary task—draining water from under the tire and ensuring grip in wet conditions. Thanks to the grooves, water is collected in these spaces, preventing loss of grip. Without sufficient grooves, the tire could not effectively expel the water, leading to hydroplaning.

The more tread grooves that make contact with the road surface, the larger the space for water collection, thereby increasing grip on slippery surfaces. The tread groove is more effective the deeper and wider it is, and the more numerous it is on the tire's contact surface. That's why winter tires usually have a diagonal pattern, known as the herringbone, which creates more "reservoirs" on the contact surface than longitudinal grooves.

Tire manufacturers specify the nominal width and number of grooves in the tread, so we have no influence on this when choosing a specific tire model. Tread groove depth, however, is closely related to tire wear. This is the only aspect we can control as users. Replacing the tire earlier improves its performance on wet surfaces, while delaying replacement significantly worsens these parameters.

To understand the importance of the number and capacity of tread grooves, it is worth looking at some motorsports where tires are cut for wet surfaces. When it starts to rain, teams cut the tires to adapt them to the new conditions at the expense of grip on dry surfaces. Cutting increases the number and width of grooves and their water capacity on the contact surface.

According to the rule described above, we are talking about a depth of 5/32 inches. Below this value, tires work significantly worse on wet surfaces. However, it is worth noting that according to regulation UN ECE-R117, which was revised and took effect on July 1, 2024, tire manufacturers are required to ensure good properties up to the tread wear limit of 2/32 inches.