That may be true, but not for the reason you state.
First, taxes have to be accounted for.
Second, how many miles per year did the average U.S. citizen drive in, say, 1930 - 1949, vs. 2000 - 2007?
OTOH, most vehicles are more efficient, now.
Per user-mile, and with taxes taken out, gasoline & diesel powered vehicles are, from the standpoint of fuel cost, a fairly good deal, at present.
This can also be seen by the shift to more trucking on the highways. I happened to be stuck in a major traffic tie-up on a rural Interstate, very recently. (A big tank truck burned, several miles in front of me.) With not a lot better to do, for almost 3 hours, I spent about 2 hours counting semi's and other large vehicles (like busses) vs. "passenger" and light duty vehicles. Almost 60% were in the "semi's" group. The truckers are close enough in cost that the other disadvantages of even "trailer trains" work against the most-of-the-route-by-rail options in many situations.
We also saw a period where a big local power plant was being supplied by coal for a while, by truck, despite it having it's own river and rail terminal on site. I was thoroughly amazed - and I bet the county road commission was swearing in its' members' sleep. The extra heavy-load traffic beat the heck out of a couple of our local 2-lane roads.
Second, how many miles per year did the average U.S. citizen drive in, say, 1930 - 1949, vs. 2000 - 2007?"
Hmmm...Excellent food for thought...{:-)