One of the best way to make vehicles more energy efficient, whether they run on gasoline, diesel, natural gas, or electricity, is to make them lighter. Aluminum is often used to do that because it can be as strong or stronger than steel while weighting a lot less. Initially, aluminum was mostly found in high-end models, like the Audi A8, but recently more of it has started to pop up in mass-market models, like the 2015 Ford 2015 F150 truck - perennially the best-selling vehicle in the US - which will be mostly made of aluminum, reducing weight by 700 pounds compared to the previous model.
To me this intuitively made sense for a few reasons:
1) All the life-cycle analysis studies that I've seen have shown that a vast majority of a vehicle's impact comes from use (ie. burning fuel) and from extracting and refining the fuel that it uses, not from manufacturing the vehicle itself. These numbers usually range from 80-90%, so anything that significantly reduces how much energy a vehicle needs to move should easily more than offset whatever extra energy is used in manufacturing.
2) About 75% of the energy used by aluminum smelters comes from hydro-power. While it's not a perfect source of energy, it certainly beats oil, so it's a good thing to swap some hydro-power in manufacturing for a reduction in gas/diesel use during use of the vehicle.
3) Aluminum is highly recyclable, and recycling aluminum uses about 95% less energy than making new one from bauxite ore. So while at first many vehicles might be made from virgin aluminum, over time most of them will be made from recycled aluminum, further improving the energy savings.
But the three points above were just my own back-of-the-envelope. Many people were still skeptical, which is always good. But now Oak Ridge National Labs has looked into the matter and seems to confirm my intuition, and they have a lot more credibility and math skills than I do.
Here is their conclusion:
They basically compared the whole life-cycle of three different versions of the same vehicle: a normal, baseline vehicle, a lightweight steel vehicle, and one that is aluminum-intensive. Their findings are that aluminum is absolutely worth it, with an energy break-even distance for their test vehicle of just 12,000 miles. That's an energy payback of just 1 year (!) for the average person, and after that all energy savings are 100% net gains over a traditional steel-based vehicle.
Oak Ridge Labs found that the aluminum vehicles has a mass about 25% lower than the baseline vehicle. This makes a pretty big difference on overall lifecycle CO2 emissions (17%):
Aluminum also corrodes less than steel, so total useful life for vehicles could be lengthened (or at least, money spent on maintenance and body work reduced). Another great thing about aluminum (and carbon fiber, which is also another great material to reduce weight without losing strength) is that it allows electric cars to have much longer range than if they were made out of steel. Below is the frame of the Tesla Model S, entirely made out of aluminum:
So it looks like there is solid evidence backing aluminum as a material of the future in the transportation sector. Over time, as cost are reduced, carbon fiber could join it to help further reduce weight and save energy.