You don't get to subtract miles traveled out of state or country or off road if you go by the odometer reading.  This is one of the tricks our benevolent politicians will use to push people over to GPS tracking.  And why would our benevolent want to push people to GPS tracking when they could make more from odometer tracking?  Because once our benevolent politicians tap into our movements via GPS tracking, they will be in heaven.  They will have struck gold.  With GPS tracking, our benevolent politicians will be able to add surcharges based on where we're driving and when we're driving.  This will make efforts like the 405 tolling look like a bunch of amateurs.