One effect of the Cash for Clunkers program is that it will have a direct and immediate reduction in the amount of fuel tax collected.
Apparently, Congress is exploring a Vehicle Mileage Tax to replace the current gas tax.
I would not mind such a change. I think it would be great to have insurance calculated per mile as well. If people were directly confronted with the cost of each trip, they might cut out unnecessary trips.
The problem, though, lies with the ability to measure miles driven. The gas tax can be charged at the point of sale, the mileage tax would have to measure the behavior of each vehicle.
Taxes that depend on measurements taken by individuals have a nasty habit of devolving into chump taxes. Measuring miles driven is likely to involve intrusive technologies like GPS units placed in each car and increased monitoring of our driving habits.
Much as I like the ideal of charging tax on mile driven, I think we are better to continue to pursue the path of charging taxes on resources consumed.