Honda BIG RED 2009 Owner's Manual page 147

Tires
Also, if you hit a hard object while driving, stop as soon as you safely
can and carefully inspect the tires for damage.
Tread Wear
wear indicator
location mark
To check the condition of a tire tread, measure the groove depth in the
center of the tire, or check the wear indicator.
For best performance, you should replace a tire before the tread depth at
the center reaches the following limits:
front
rear
138 Servicing Your Honda
groove depth
3/16 in (4 mm)
wear indicator
3/16 in (4 mm)
3/16 in (4 mm)