Basically, by combining GPS units that tracks miles driven but only transmit the data to receivers at gas stations and instant tax calculation at the pump, Oregon managed to eliminate a lot of the worries that opponents have of a mileage tax. These include privacy issues and added complexity. Still, one of the biggest problems with a mileage tax - that fuel-efficient vehicles and gas guzzlers are treated equally, thereby eliminating at least one incentive for people to shift to vehicles that burn less fuel - remains, no matter how well the system works. What's more, participants in the program ended up driving less than a control group, so there were some environmental benefits. If you're in favor of charging taxes by the mile, take a look at how one state managed to implement the tax without offending the (self-selected) people paying the money.
Photo by JeffWilcox. Licensed under Creative Commons license 2.0.