I've been thinking about this lately, I've concluded that the only real measure is fuel used per unit time.  It does not matter HOW you achieve it, the goal should be to reduce the number of gallons you use, say, in a month.  If that be from a higher mpg car, or combining trips, or car pooling, or walking, or taking the train, whatever.
Bingo!  We have a winnah!

This is the point I keep making to people in person--don't get caught up in the minutiae of the situation, just look for ways to minimize your monthly energy expenses.  Use less (turn off lights in unused rooms, batch errands together, car pool), and use energy more efficiently (use CFL's, drive with a lighter foot), and you can save a healthy percentage of your energy bill, beginning right now, without making a major investment or change in your lifestyle.