As cars get more efficient in terms of gas use, the gov't wallet slims down.. but given the same car in terms of e.g. weight, footprint (literal - i.e. tires-on-road), it doesn't matter whether you're super-efficient or the worst gas guzzler in the world... you're still putting the same wear-and-tear on that road. Ergo, they have to..
Actually, no, they don't. More fuel efficient cars are, on average, lighter so they do not put more wear and tear on the road, they put less on it. As fuel efficiency becomes more important, you should see vehicle weights drop. We did in the 70's and we are seeing it now (SUV vs Prius or Land Rover vs CR-V if you want apples to apples). I'm inclined to believe that the solutions to reduced tax revenue due to higher fuel efficiency is to cut government spending but that's not going to happen. A Gas tax is simple, straight forward, hard to loophole (verb?) and reasonably fair. Is it perfectly fair? Probably not but what is? Creating new taxes will likely do more to create additional bureaucracy than increase revenue.
Personally, I'd favor higher gas tax to reduce income tax. Why punish people for working or success? If we are going to have to pay, make it for something that we would rather not be spending our money on in the first place. Higher gas prices would drive greater fuel efficiency and "fuel" the development of alternate energy sources. Being a unique state to tax miles driven will just make people want to drive elsewhere. It also fails to achieve the ultimate goal of taxation (queue Monty Python) Tax all foreigners living abroad. It becomes a tax for the locals to bear.
Finally, this is just a great example of how Americans can deal with a problem by becoming more efficient. When fuel costs go up, we get more efficient cars, carpool, telecommute. It is also a great example of the American government failing to deal with a problem or dealing with it by becoming less efficient.
HOLY MACRO!