The year is 2020 and the gasoline tax is history. In its place you get a monthly tax bill based on each mile you drove — tracked by a Global Positioning System device in your car and uploaded to a billing center. What once was science fiction is being field-tested by the University of Iowa to iron out the wrinkles should a by-the-mile road tax ever be enacted. Besides the technological advances making such a tax possible, the idea is getting a hard push from a growing number of transportation experts and officials.