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The good news is that the average fuel fuel economy of the entire US light-duty fleet improved by 40 percent over the past four decades (increasing from 13 miles per gallon to 21.6 mpg). The bad news is that Americans drive more, and with fewer passengers in each vehicle, undercutting the impact of the fuel economy gains.

That's the official conclusion of a study just released by the Sustainable Worldwide Transportation at University of Michigan Transportation Research Institute (UMTRI). Without getting too technical, the researchers found that the "occupant fuel economy" (a figure that takes mpg and vehicle occupancy into account) dropped as vehicle loads decreased by 27 percent (from 1.9 to 1.38 persons, on average) over the years. Combine the additional distance driven, up a whopping 155 percent, and that average fuel economy improvement of 40 percent drops to an occupant fuel economy gain of just 17 percent.

So what does this all mean? Saving fuel on a grand scale is more than just bumping fuel economy on the cars in the showroom. Consumers will need to drive more efficient vehicles, but they also must drive less and learn how to carpool. Easier said than done.