I think there is some confusion over declination and variation.
Magnetic deviation is the difference between where the compass points, and
true north. This is a function of relative location compared to the
isogonic line which runs thru western TN and southward. Some areas of
Alaska hve as much a 45� easterly declination. Whereas, Nashville,TN has
almost none, as it falls along the isogonic line. The declination may
require recalibration as one drives great distances. I think I have a jpeg
of the deviation map for US if anyone is interested.

The magnetic variation is what is induced by local anomolies such as the
car body, bridges etc.
Cheers,

Dave C.
Williamsburg, VA
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'85 GL Westy (His)
'91 Cabriolet (Hers)