Traverse City, Michigan—The change in consumers’ car choices over the last few months has rocked every automaker, import as well as domestic. Toyota’s Bob Carter illustrated the change yesterday with a single, stark statistic: In May and June, more than 50 percent of the vehicles sold in the US were fitted with four-cylinder engines—for the first time in history, or at least since World War Two.

But lost amidst talk of hybrids, plug-in hybrids, full electric vehicles, and techniques like gasoline direct injection, is a simple fact of physics: The greater the weight, the more energy it takes to move.

Or put another way, cutting vehicle weight saves gas. And despite idealistic notions of an entire nation driving Honda Fits, it won’t happen—too many people really do need midsize, full-size, or truck-based vehicles.

So what’s the actual impact of cutting weight? On Tuesday, Michael Bull, director of technology at Novelis Corporation, presented data from two studies that quantified the fuel savings from a given weight reduction.

The first study, conducted by respected British design and consulting firm Ricardo, looked at the fuel savings in different vehicle classes from reducing weight by 10 percent—while keeping performance comparable. The study showed that a 10 percent weight reduction would cut fuel consumption 3 to 4 percent using the same engine. But the savings rose to 6 to 7 percent if the engine was downsized for the lighter weight—and those numbers stayed constant across most vehicle classes, and applied equally to gasoline and diesel engines.

Aluminum is the traditional way to cut weight from a metal body structure. For a midsize car, making “closure panels” (doors, hood, trunk) out of the lighter metal can save 140 pounds or about 4 percent. Substituting aluminum for major parts of the car’s “body in white,” or base structure, takes savings to 470 pounds, or 12 percent—and downsizing the drivetrain for the lower weight brings the total reduction to 17 or 18 percent. That improves fuel consumption by 11 or 12 percent.

A second study, from Ibis, then compared the cost-effectiveness of different tactics for reducing weight, comparing gasoline, diesel, and hybrid drivetrains in both steel and aluminum vehicles. With the steel vehicle as the baseline, increasing the aluminum content of the structure cost $44 per unit of mileage—but adding hybrid drive to the steel vehicle cost $259 per mileage unit, with a diesel engine coming in at $192. Combining an aluminum structure with the hybrid drive reduced the cost per mileage unit to $173, and it fell further to $104 with a diesel engine in the aluminum structure.

In other words, cutting weight may be a more cost-effective way to save fuel than adding hybrid-electric drive. In reality, new fuel-economy regulations and carbon constraints will most likely encourage automakers to combine several techniques in the same vehicle.

But it’s noteworthy that Ford—whose CEO formerly worked at Boeing, where every ounce mattered in its products—has cited weight reduction as one of its five core strategies for sustainability. Even more significant, last month Toray Industries, Nissan, and Honda agreed to work together on developing carbon fiber for auto bodies, with the goal of replacing most of the steel in a car body—for a projected weight savings of 40 percent.