New indicator: when the number 4 disappears, the tire is due for replacement

Tread depth plays a vital role in tire performance, particularly on wet surfaces and snowy conditions for winter tires. Thus, tire manufacturers use the Tread Wear Indicator (TWI) to keep users informed about their tire's wear degree. An innovative alternative has emerged in the form of a different tread depth indicator, now employed by a few tire manufacturers.

Markings of this variety first made their appearance on Nokian tires, and have since been adopted by companies like Continental and Goodyear. Now, many other manufacturers feature these notations on their tires. These numbers merely denote the tread groove depth in millimeters, with each number corresponding to a specific depth. As the tire wears, respective numbers disappear.

The lowest visible number is 4. For good reason, as a tread depth of approximately 0.16 inches doesn't grant the needed attributes for winter or summer tires. The presence of only the number four is a sign that your tire still possesses some usability, but also serves as a cue to consider its replacement. Once the number 4 vanishes, the tire is ripe for replacement.

Tread grooves provide a storage area for water where the tire interfaces with the road. These grooves hold the water when driving on a wet surface and act as a reservoir for water left on the road that hasn’t been displaced by the tire. In the absence of these grooves, a smooth tire wouldn't be able to displace the surface water, resulting in a loss of vehicle traction.

The greater the number of tread grooves on the tire's contact area with the road, the more space there is for water, and thereby, the more storage capacity. The width and depth of these grooves, and their quantity on the tire, determine their capacity. Winter tires often bear slanted tread patterns or 'zig-zags' to accommodate a larger number of grooves on the tire's road contact point. This is why the contact area between the tire and a road resembles a rectangle.

The tire manufacturer dictates the nominal width of the grooves and their number, so we can't influence that after choosing a specific model. However, the groove depth depends solely on the tire wear and is the only parameter users can control. Replacing the tire earlier will improve its performance on wet roads, but delaying it through the grooves wearing out will degrade its performance markedly.

To grasp the importance of the number or capacity of the tread grooves, one can look to various motorsports disciplines where tire slitting is practiced. When rain starts, teams cut tire tread during service, adapting it for wet or muddy conditions, albeit compromising dry-surface traction. The focus remains on enhancing the grooves' overall capacity on the contact area with the road.