As an example only - I live in Pennsylvania, obviously. But I often do field work outside of the state. I once worked for 3 1/2 months in Georgia, and put over 12,000 miles on the vehicle during that assignment - pretty much all of it outside of Pennsylvania.

Yet by taxing miles, I'd be taxed for all 12,000+ of it even though most of it was not in Pennsylvania. This is not exactly a good solution either.