GMC Sierra 2005 Owner's Manual page 150

Compass Variance
The mirror is set in zone eight upon leaving the
factory. It will be necessary to adjust the compass to
compensate for compass variance if you live outside
zone eight. Under certain circumstances, as during
a long distance cross-country trip, it will be necessary
to adjust for compass variance. Compass variance is the
difference between earth's magnetic north and true
geographic north. If not adjusted to account for compass
variance, your compass could give false readings.
To adjust for compass variance:
1. Find your current location and variance zone
number on the following zone map.
2. Press and hold the COMP button for five seconds
until a zone number appears in the display.
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3. Press the COMP button on the bottom of the mirror
until the new zone number appears in the display.
After you stop pressing the button, the display
will show a compass direction within a few seconds.