Ford 2005 F250 Owner's Manual page 138

Tires, Wheels and Loading
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 mm), 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 mm).
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.
Age
Tires degrade over time, even when they are not being used. It is
recommended that tires generally be replaced after 6 years of normal
service. Heat caused by hot climates or frequent high loading
conditions can accelerate the aging process. You should replace the
spare tire when you replace the other road tires due to the aging of
the spare tire.
U.S. DOT Tire Identification Number (TIN)
U.S. and Canada Federal regulations require tire manufacturers to place
standardized information on the sidewall of all tires. This information
identifies and describes the fundamental characteristics of the tire and
also provides a U.S. DOT Tire Identification Number for safety standard
certification and in case of a recall.
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