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Cars' Big Weight Gain Hurts Fuel Economy, Study Says

If ever there was a perfect illustration of how cars got to be so huge, this is it, and from eco-conscious carmaker Honda! In 20 years, the Honda Accord has not only added 50 percent to its weight, it has also gone on steroids--increasing horsepower by a factor of 2.5.

This is hardly an isolated incident. According to Christopher Knittel of the Institute of Transportation Studies at the University of California, Davis (whose work the chart represents), "From 1980 to 2004 the average fuel economy of the US new passenger automobile fleet increased by less than 6.5 percent. During this time, the average horsepower of new passenger cars increased by 80 percent, while the average curb weight increased by 12 percent."

The author adds that if we just stopped adding so much weight, cars today would be much more fuel efficient. "[I]f weight, horsepower and torque were held at their 1980 levels," he said, "fuel economy for both passenger cars and light trucks could have increased by nearly 50 percent from 1980 to 2006; this is in stark contrast to the 15 percent by which fuel economy actually increased."

Wow, all cars had to do was stop putting on weight. But, as we humans know, slimming down is easier said than done. The Obama Administration recently adopted Corporate Average Fuel Economy (CAFE) standards calling for cars to get 35.5 mpg by 2016, but weight gains are the enemy of that. Knittel adds, "I also find that once technological progress is considered, meeting the CAFE standards adopted in 2007 will require halting the observed increases in weight and engine power characteristics, but little more; in contrast, the standards recently announced by the new administration, while certainly attainable, require non-trivial 'downsizing.'"

In other words, cars will have to get on a scale. As I wrote here last week, the stripped-down Nissan Versa I'm test-driving (with roll-up windows, no radio, and power nothing) points a way toward a possible car of the future. For every action there is an equal and opposite reaction: Adding features and power also inevitably adds weight, and our future cars, either powered by electric motors or smaller gas engines, will need to watch every pound.

First chart from: Knittel, Christopher R. (2009) "Automobiles on Steroids: Product Attribute Trade-Offs and Technological Progress in the Automobile Sector." Institute of Transportation Studies, University of California, Davis, Research Report UCD-ITS-RR-09-16. Second chart from U.S. Bureau of Transportation Statistics.