There's a lot of focus--perhaps too much--on carmakers using smaller engines, hybrids, and plug-in cars to meet increasingly higher gas-mileage standards from now through 2025.
But another, equally important method is reducing the amount of car that has to be moved around in the first place.
That doesn't mean making cars smaller--absent a significant and sustained rise in the price of gas, the mix of cars and trucks sold changes very slowly, if at all--but making them lighter.
It's the invisible way to boost gas mileage. And two recent introductions typify the trend.
First, the all-new 2013 Range Rover--just unveiled last night--that will go on sale in December.
It's only the fourth all-new Range Rover since the iconic luxury all-wheel drive brand launched in 1970.
The new model, while stuffed with more electronics and luxury features than ever, is up to 925 pounds lighter than the outgoing model.
It uses an all-aluminum unibody structure that weighs fully 40 percent less as a "body in white" than did the steel shell of the last model. Front and rear subframes and suspension carriers are aluminum too, supporting air suspension for all four wheels.
Range Rover says the U.S. model, with a 5.0-liter gasoline V-8 engine, weighs 700 pounds less than a similar outgoing model. In Europe, there are two V-6 diesel options too. (Despite a prototype for a model intended to launch in 1999, no V-12 Range Rover is likely to be built now or in the future.)
2013 Cadillac ATS
The second case study is the 2013 Cadillac ATS, which GM's luxury brand is putting directly up against that icon of compact sports sedans, the BMW 3-Series.
Cadillac is very proud of the fact that the ATS is 58 to 146 pounds lighter than a comparable four-cylinder BMW 328i--and likely lighter than it would have been if designed even four years earlier.
Unlike the Range Rover, the ATS uses a steel body structure. But it uses a variety of high-strength steels and carefully engineered structure to eliminate unnecessary mass while keeping the desired performance characteristics.
2013 Cadillac ATS
As Ward's Auto notes, GM engineers learned a great deal about minimizing weight on the ATS program that they will carry over to future vehicle programs.
And there's more to come on the weight-saving front. The 2014 BMW i3, its first all-electric car, uses a body structure of Carbon Fiber-Reinforced Plastic (CFRP) that sits on an aluminum platform that carries the battery, electric motor, and power electronics.
Expect to see more aluminum, a slow emergence of carbon fiber for car bodies, and far, far more high-strength steel and complex structural engineering in cars to come.
It takes less energy to move a lighter object the same distance. That's basic physics.
There's also aerodynamics, another (mostly) invisible way to reduce the energy lost to wind resistance at speeds of 40 mph or above.
But we'll leave that one for another day.