In reply to codrus (Forum Supporter) :
I think it depends on the state. In PA, a mileage tax wouldn't be terribly hard to implement due to cars requiring annual inspections where the mileage is already recorded by the inspection station. All that would be required would be a program that took inspection data and spit out a bill to the car owner.
From the technical side - fairly easy and even though there are ways to fudge the system, enough people would follow the rules that making it too complicated wouldn't be worth the extra expense. Now from the political side - that's a different story and I don't see something like this getting through the PA legislature easily.
My toy cars already get the highest octane gas I can find for them, so I'm already used to paying more to feed them.