GMC 2002 Safari Owner's Manual page 125

Variance is the difference between magnetic north and
geographic north. In some areas, the difference between
the two can be great enough to cause false compass
readings. If this happens, follow these instructions to set
the variance for your particular location:
1. Find your location on the zone map. Record your
zone number.
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2. Press and hold both the MODE and the US/MET
buttons in the COMP/TEMP mode.
3. After five seconds, the compass will acknowledge
the variation mode by displaying the current zone
number. When it does, release both buttons.
4. Press US/MET until your zone number appears on
the display.
5. Press MODE to enter your zone number. Your
variance is now set and the display will return to the
COMP/TEMP mode.