Chevrolet Silverado 2007 Owner's Manual page 177

The compass can be calibrated by driving the
vehicle in circles at 5 mph (8 km/h) or less until
the display reads a direction.
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 the vehicle is 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, the 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 on/off button until a Z and
a zone number appears 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.
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