1998 GMC Suburban Owner's Manual - Page 124

The mirror can be calibrated in one of two ways: Drive the vehicle in circles at five mph (8 km/h) or less until the display reads a direction, or Drive the vehicle on your everyday routine. 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 forcompass variance: 1. Use the COMP button located at the bottom of the mirror. 2. Press and hold the COMP button for three seconds until a zone number appears in the display. 4. 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 in, the display will show a compass direction within a few seconds. 3. Find your current location and variance zone number on the following zone map. 2-46