We've noted before that cars and trucks in the United States have starting becoming more fuel-efficient in recent years — after decades of stagnation — in response to high oil prices and strict new vehicle standards. That's one reason why U.S. oil imports have plummeted.

But a new report (pdf) from the Environmental Protection Agency offers the most detailed breakdown yet of this trend. The EPA is a particularly helpful source on fuel-economy because the agency tests how cars and trucks actually perform in the real world — rather than simply looking at what the laws say.

Here's what the EPA found:

1) Fuel-economy for new cars and trucks has recently hit a record high, after decades of stagnation. New cars and trucks sold in the United States have become more efficient since 2004 — after having slipped during the 1980s and 1990s. (Do note, however, that last year's fleet-wide average of 23.8 miles per gallon is still far, far less than the fuel economy seen in places like Europe and Japan):

2) Smaller cars are now making a comeback after years of increasing light-truck and SUV dominance. Note that the recent blip was likely due to the 2011 tsunami, which temporarily throttled the availability of Japanese cars:

3) Hybrids are catching on, but most of the gains so far have come from a wide variety of tweaks to combustion engines. An example: gasoline direct injection, an efficient technique for delivering fuel to a car's engine. That said, hybrids and electric cars are expected to drive big gains in fuel-economy in the coming decades, as new cars will need to average more than 35 miles per gallon on the road by 2025:

4) Cars are no longer getting heavier. In the past, automakers would use advances in engine technology to build bigger, more powerful cars. That's no longer the case. U.S. vehicles are now getting more powerful and more fuel-efficient without adding weight.

5) It's not just Japanese cars. American automakers are building more fuel-efficient cars too. Last year, I wrote about how Detroit learned to build cars that got decent gas mileage. The EPA report brings the data, noting that all companies are adapting to the era of high oil prices:

(Note that Kia and Hyundai aren't on here because they're under EPA investigation for allegedly misstating their fuel-economy numbers. Toyota and Honda saw a dip in 2011 because of supply-chain issues related to the tsunami.)

Note that these fuel-economy trends are expected to accelerate in the coming decades. The Obama administration has set new rules that will require cars and light trucks to average 54.5 miles per gallon by 2025. (Under EPA measurements, that will likely translate into around 35.4 miles per gallon on the road.) So there's still a long ways to go.

Further reading:

--Even with strict new fuel-economy standards, the U.S. is still lagging behind places like Europe and Japan.

--How Detroit figured out how to build smaller, efficient cars that people would buy.

--Hyundai, Kia overstated their fuel-economy numbers. How often does this happen?

--As battery prices drop, will electric cars finally catch on?