Chevrolet 2008 Tahoe Owner's Manual page 151

In order to calibrate, CAL must be displayed in the
mirror compass windows. If CAL is not displayed, push
in the compass/temperature button for approximately
nine seconds or until CAL is displayed.
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 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 compass/temperature button
for six seconds until a zone number appears in
the display.
3. Press the compass/temperature 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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