FORD 2013 F-550 v.2 - Manual (Page 359)

Improper or inadequate vehicle maintenance can cause tires to wear
abnormally. Inspect all your tires, including the spare, frequently, and
replace them if one or more of the following conditions exist:
Tire Wear
When the tread is worn down to
1/16th of an inch (2 millimeters),
tires must be replaced to help
prevent your vehicle from skidding
and hydroplaning. Built-in
treadwear indicators, or wear bars,
which look like narrow strips of
smooth rubber across the tread will
appear on the tire when the tread
is worn down to 1/16th of an inch
(2 millimeters).
When the tire tread wears down to the same height as these wear bars,
the tire is worn out and must be replaced.
358 Wheels and Tires
2013 F-250/350/450/550 (f23)
Owners Guide gf, 2nd Printing, December 2012
USA (fus)