+1 - just
raise the damned gas tax! The federal tax is currently 18.4 cents per gallon, and hasn't been raised since 1993.
Raising it would be a "free market" solution to many problems (VMT, average fuel economy, global warming, even obesity if people started actually (gasp!) walking or riding their bikes to the grocery store). They could call it a "War on Terror Tax", or a "Freedom Tax", or a "Carbon Tax", or whatever.
Unfortunately, neither political party has the balls to propose something like this. So, we'll probably just keep subsidizing SUV's and Hummers, and sucking at the Saudi teat until it runs dry.
Looking at the replies here, it is pretty obvious that would be a difficult thing... doing the tax as a percentage however would have the same effect and actually increase as the price of fuel went up. (so further adjustments would not have to be made in the future.)
Now consider the way to gain support for such a tax would be to offer rebates for using alternate transport. A "carrot" to go with the "stick" of the tax for fuel use.