2008 Chevrolet 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. To adjust for compass variance: 1. Find your current location and variance zone number on the following zone map. 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. 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. 2-53