So, you've done the right thing: You've decided your next car will be considerably more fuel-efficient than the one you drive now.

That's great news. But, have you considered how it's built--and more to the point, what it's built out of?

A new peer-reviewed paper from the Oak Ridge National Laboratory suggests that aluminum could be the best material from which to make cars and trucks.

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That's because, the paper says, the reduction in carbon emissions from using less fuel to propel a lighter vehicle over its lifetime more than offsets the higher emissions generated by using aluminum rather than conventional steel.

Historically, only large luxury vehicles had used full aluminum bodies or structures. Among them are the Audi A8, Jaguar XJ, the Range Rover and Range Rover Sport, and the Tesla Model S.

With the 2015 Ford F-150 set to boost annual production of aluminum-bodied vehicles starting later this year, the energy impact of using the lighter metal comes under increasing focus.

Back in 2000, a study by M.A. Weiss et al.,  On the Road in 2020: A Lifecycle Analysis of New Automotive Technologies, calculated that 75 percent of a vehicle's lifetime carbon emissions came from its use.

A further 19 percent of its lifetime carbon emissions stemmed from the extraction, refining and transportation of the fuel it uses.

Aluminum Cubed [Image by Flickr user jpeepz]

Aluminum Cubed [Image by Flickr user jpeepz]

That meant just 6 percent of a car's lifetime carbon emissions were due to the extraction and production of raw materials, and the production of the vehicle itself.

But as cars become more efficient, that balance changes.

If a car spends its 15-to-20-year life consuming far less fuel, the raw materials and assembly--where energy use changes less--accounts for a greater proportion of its lifetime carbon footprint.

Just last month, Volkswagen suggested that production of its Golf TDI Bluemotion diesel accounts for fully 22 percent of its lifetime carbon emissions, with 9 percent for fuel extraction, refining, and transport, and 68 percent from burning that fuel.

Overall, thanks to high fuel efficiency and low emissions, greenhouse gas emissions per vehicle are falling. Proportionally, though, production of the vehicle itself becomes much more significant.

"Best choice for the environment"

Aluminum turns out to be quite energy-intensive to use in vehicles.

It's extracted from bauxite in much smaller quantities than the iron used in steel, and requires huge amounts of energy to process.

This means the typical aluminum vehicle has a much larger carbon footprint than a regular vehicle at the outset.

However, aluminum offers several advantages when used in car construction.

2015 Land Rover Range Rover Sport Hybrid, 2013 Frankfurt Auto Show

It's much lighter than steel, with no compromise in strength. Cutting weight means better performance, handling, and fuel economy--all qualities desirable in everything from supercars to tiny electric vehicles.

It's also much less corrosive than steel in typical atmospheric conditions, potentially giving aluminum cars a longer lifespan. Aluminum will corrode, but the microscopic layer of aluminum oxide that forms on its outer layer prevents further corrosion like the rampant rust seen in older steel vehicles.

The Oak Ridge report suggests manufacturing should account for 10 percent of an average aluminum vehicle's lifetime carbon emissions.

That's higher than the 2000 MIT report, but lower than VW's recent estimates for the high-efficiency diesel Golf.

Importantly though, Oak Ridge calculates that aluminum cuts total lifecycle energy consumption in a typical U.S. crossover utility vehicle by 20 percent, producing a carbon footprint 17 percent lower.

Environmental payoff in one year of driving

At that rate, it would take just 12,000 miles for an aluminum-intensive vehicle to offset the extra energy consumed to built it. For many U.S. vehicles, that's just a year of use or less.

It comes down to math: Even as cars get more efficient, the largest proportion of their energy use and emissions remain associated with the fuel they consume.

Provided that fuel consumption falls notably, a higher energy use in production can be comfortably offset over a car's life.

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