GMC 2007 Sierra - Manual (Page 182)

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.
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.
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