Chevrolet Silverado Classic 2007 Owner's Manual page 175

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 of 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 do the following:
1. Find your current location and variance
zone number on the following zone map.
2. Press and hold the on/off button until a Z and
a zone number appear in the display. The
compass is now in zone mode.
3. Keep pressing the on/off button until the
desired zone number appears in the
display. Release the button. After
approximately four seconds of inactivity, the
new zone number will be locked in and
the compass/temperature display will return.
4. Calibrate the compass as described next.
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