2008 GMC Sierra 1500 Pickup Owner's Manual - Page 114

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 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. 4. Calibrate the compass as described below. 2-38