Why cars will keep getting lighter

At this year’s Detroit auto show, there’s been a lot of hoopla over fancy new electric cars and hybrids. Less-remarked has been the fact that automakers are shifting to smaller and lighter vehicles. With good reason: This is, by far, the easiest way to boost gas mileage.

(STAN HONDA/AFP) A new book by Amory Lovins and the Rocky Mountain Institute, “Reinventing Fire,” lays out some striking stats on this. A car’s weight is responsible for about two-thirds of the energy needed to move it, with the energy required to move a vehicle increasing roughly in proportion with its heft. Heavier autos also need bigger engines to achieve the same acceleration, and those engines use, on average, just 8 percent of their power in highway driving and 5 percent in the city — a big reason why hulking cars get such poor fuel mileage.

So slimming cars and SUVs down is a simple way to boost fuel economy. Lighter steel and better designs have been bringing the weight of cars down dramatically, without hiking the pricetag. (Check out the FutureSteelVehicle project which suggests that cars could get 35 percent lighter simply with better steel techniques.) This is a significant advance: Lovins calculates that cars could boost fuel economy by a staggering 50 percent just by shedding weight, improving drag, and using low-rolling-resistance tires. That’s without even mentioning electric hybrids or plug-in vehicles. (This is also before any discussion of advanced composite materials like carbon fibers, which have the potential to reduce weight even further, but which are still pretty pricey.)

Similarly, MIT economist Christopher Knittel has pointed out (PDF) that auto manufacturers could quite easily meet the new CAFE standards for fuel economy — under which they’ll have to bring their fleet averages up to 54.5 miles per gallon by 2025 — if engine innovations simply advanced at the same rate they did between 1980 and 2006, but cars slimmed down. (Most of the improvements in engine technology in the past few decades have gone toward making heavier, more powerful vehicles rather than toward boosting gas mileage.)

Of course, there’s also the question of safety — a topic that came up in comments the last time we discussed this issue. The old conventional wisdom was that heavier cars were safer — thanks to old studies done by the National Highway Traffic Safety Administration, starting in 1977, which suggested (PDF) that making cars 100 pounds lighter would kill an extra 400-1,300 Americans per year. These studies helped encourage a shift to heavier and heavier cars.

But it turns out the analysis had confused “weight” with “size.” Those are two separate things. It’s true that larger cars are safer, because they have more space to absorb the impact of a crash. But the weight of a car isn’t particularly important here. Indeed, as Lovins points out, “across all cars on U.S. roads, observed crash death rates vary by about threefold between different models of the same weight.” Smart design is much more important for safety. Weight, by contrast, mostly just affects fuel economy. And that’s why, in an age of rising gas prices, automakers are getting particularly interested in putting their cars on diets.