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Cars in the U.S. are more fuel-efficient than ever. Here’s how it happened.

Cars and light trucks sold in the United States hit a new record for fuel efficiency last year — 23.6 miles per gallon, on average — in response to still-high oil prices and strict new fuel-economy standards.

That's a big step up from the 22.4 miles per gallon average for new vehicles in 2011. And overall fuel economy is expected to increase to 24 miles per gallon in 2013, another record.

How did this happen? A new report from the Environmental Protection Agency offers a detailed breakdown. The EPA is a particularly helpful source on fuel efficiency because the agency tests how cars and trucks actually perform in the real world — rather than just looking at what the fuel-economy rules dictate should happen. Here are some highlights:

1) Fuel economy for new cars and trucks hit a record high in 2012:

New cars and trucks sold in the United States have become steadily more efficient since 2004 — after fuel economy stagnated during the 1980s and 1990s.

So what changed? Oil prices began their dramatic rise around that time. That pushed many drivers to buy more fuel-efficient vehicles. Then, in 2007, Congress enacted a new round of fuel-economy standards, the first time those requirements had been tightened since 1984. The Obama administration later raised those fuel-economy goals even higher.

Do note, however, that last year's U.S. fleet-wide average of 23.6 miles per gallon is still fairly meager compared to the fuel economy in places like Europe and Japan.

2) Smaller cars have slowly made a comeback after years of increasing light-truck and SUV dominance. 

Note that the blip in 2011 was likely due to the earthquake, tsunami, and nuclear accident in Japan, which temporarily throttled the availability of Japanese cars.

3) Hybrids are expanding their market share, but automakers are still largely focused on tweaks to combustion engines. 

The chart above shows the different types of fuel-saving technologies that have become more prevalent since 2008. That includes everything from hybrids to combustion-engine technologies that are decades old — such as "variable valve timing" or multi-valve engines — that are still expanding.

As the chart shows, tweaks to combustion engines have been a major focus in the past five years.* One example: gasoline direct injection, a more efficient technique for delivering fuel to a car's engine. That said, hybrids and electric cars are expected to drive bigger gains in fuel economy in the decade ahead, as new cars will need to average more than 35 mpg on the road by 2025:

4) Cars are no longer getting heavier — and that's made a huge difference.

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 listed here because they're still under EPA investigation for allegedly misstating their fuel economy numbers. Also, Toyota and Honda saw a dip in 2011 because of supply-chain issues related to the tsunami that year.)

These fuel economy trends are expected to continue 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 to around 35.4 miles per gallon on the road.) So there's still a long way to go.

Clarification. Changed the wording here to make it more accurate. The EPA report doesn't say how much each engine technology is contributing to fuel-efficiency gains.

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?