GMC Sierra 2000 Owner's Manual page 30

Adjust the height so that the shoulder portion of the
belt is centered on your shoulder. The belt should be
away from your face and neck, but not falling off
your shoulder.
Q:
What's wrong with this?
A:
The shoulder belt is too loose. It won't give nearly
as much protection this way.
CAUTION:
You can be seriously hurt if your shoulder belt is
too loose. In a crash, you would move forward
too much, which could increase injury. The
shoulder belt should fit against your body.
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