Check tread depth.

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Minimum depth: 1.6 mm

NOTE:

This value may vary for individual countries due to different legislative regulations.

Fig 1: Checking Tire Tread Depth

Courtesy of VOLKSWAGEN UNITED STATES, INC.

The minimum tread depth is reached when the tires have worn down level with the 1.6 mm high tread wear indicators - arrows - positioned at intervals around the tire.
If the tread depth is approaching the legal minimum permissible depth, the customer must be informed.