For decades, the United States has been the land of the gas guzzler. But that's changed significantly over the past five years.

New vehicles purchased in November of 2012 got 24.1 miles per gallon, on average. That includes all new cars, SUVs, and light trucks. By contrast, new vehicles averaged just 20.1 miles per gallon average back in the fall of 2007.

Those numbers come from Michael Sivak of the University of Michigan, who also passes along this chart showing that newly purchased cars and light trucks in the United States are polluting far less than they did five years ago:

New cars bought in the fall of 2012 are using about 15 percent less fuel per mile than cars purchased in 2007. But they're also logging slightly fewer miles overall — a sign that Americans aren't just negating the fuel savings by driving more. Add it all up, and there's been a 20 percent drop in greenhouse-gas emissions from new vehicles in the past five years.

How much of a difference does this make in the grand scheme of things? Brand-new vehicles, after all, are only a small portion of the overall U.S. fleet. Yet in a recent paper, Sivak and Brandon Schoettle estimated that recent efficiency upgrades have already reduced carbon-dioxide emissions from all U.S. light-duty vehicles by about 2.9 percent.

Combine that with the fact that Americans have been driving fewer miles overall, and that's a small but real improvement as far as oil use and climate change are concerned.

This trend is likely to continue. The Obama administration has already set strict new fuel-economy standards for cars and light trucks, which are expected to average around 35.4 miles per gallon by 2025. Indeed, this is one big reason why U.S. oil imports have been shrinking — along with increased domestic oil production — and it's why imports are projected to keep shriveling in the decades ahead.