James Glassman points out that increased fuel efficiency in our vehicles will most likely increase our dependence on foreign oil:

In this graph, the authors [Peter Huber and Mark Mills] show how the "energy cost" of transportation in the U.S. fell by nearly one-third between 1973 and 2003; that is, we used to use nine gallons of fuel for every vehicle mile, now about six. But over this same period, total fuel use did not drop by one-third (as it would under the silly static analysis employed by Mineta and Zakaria); instead, fuel use rose by more than half, from a little under 120 billion gallons per year to over 180 billion gallons.