FORD 2013 F-550 v.3 - 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.
Damage
Periodically inspect the tire treads and sidewalls for damage (such as
bulges in the tread or sidewalls, cracks in the tread groove and separation
in the tread or sidewall). If damage is observed or suspected, have the tire
inspected by a tire professional. Tires can be damaged during off-road use,
so inspection after off-road use is also recommended.
WARNING: Age
Tires degrade over time depending on many factors such as
weather, storage conditions, and conditions of use (such as load,
speed, inflation pressure) the tires experience throughout their lives.
In general, tires should be replaced after six years regardless of tread
wear. However, heat caused by hot climates or frequent high loading
conditions can accelerate the aging process and may require tires to be
replaced more frequently.
You should replace your spare tire when you replace the road tires or
after six years due to aging even if it has not been used.
358 Wheels and Tires
2013 F-250/350/450/550 (f23)
Owners Guide gf, 3rd Printing, December 2012
USA (fus)