GMC 1996 Sierra Owner's Manual page 302

When It's Time for New Tires
Rust or dirt on a wheel, or on the parts to which
it is fastened, can make- wheel nuts become loose
.after a time. The wheel could come off and cause
an accident. When you change a wheel, remove
any rust or dirt from places where the wheel
attaches to the vehicle. In an emergency, you can
use a cloth or a paper towel to do this; but be
sure to use a scraper or wire brush later, if you
need to, to get all the rust or dirt
off.
(See
"Changing a Flat Tire" in the Index.)
One way to tell when it's
time for new tires is to
check the treadwear
indicators, which will
appear when your tires have
only 1/16 inch (1.6 mm)
or
less of tread remaining.
Some commercial truck
tires may not have
treadwear indicators.
Its
You need a new tire if any of the following statemen
are true:
0
You'can see the indicators at three or more places
around the tire.
You can see cord or fabric showing through the tire's
rubber.
0
The tread or sidewall is cracked, cut or snagged deep
enough to show cord or fabric.
0
The tire has a bump, bulge or split.
0
The tire has a puncture, cut or other damage that
,
can't be repaired well because of the size or location
of the damage.