See also:

Treadwear indicators
Narrow bars (tread wear bars) that are distributed over the tire tread. If the tire tread is level with the bars, the wear limit of 1/16 in (1.6 mm) has been reached. ...

Mercedes-Benz M-Class Owners Manual
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Attaching the roof carrier
► Secure the roof carrier to roof rails 1. ► Observe the manufacturer's installation instructions. ...