Here at GreenCarReports.com, we read a lot. And increasingly, our friends send us stuff they think we'd be interested in. Which is how we came across an abstract of a study done by one Christopher R. Knittel at the University of California-Davis.

Knittel looked at the factors affecting carmakers' corporate average fuel economy (CAFE) from 1980 to now, in particular the tradeoffs among engine power, vehicle weight, and gas mileage. His conclusion?

The results suggest that if weight, horsepower and torque were held at their 1980 levels, 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.

In other words, automakers made cars and trucks both faster and heavier, so gas mileage increased only 15 percent; if they'd kept weight and performance constant, mileage could have risen 50 percent.

Of course, much of that weight gain is due to added safety equipment (e.g. stronger structures, multiple airbags) and more features (e.g. electric seats, multi-speaker stereo systems). Consumers like those things; they like quicker acceleration, too.

There's more: 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But those regulations, from the Bush Administration, have been superseded by the more aggressive CAFE standards announced in May by the Obama Administration. Those standards, says Knittel, "while certainly attainable, [will] require non-trivial 'downsizing'."

We're not sure we agree with Knittel on that last point. We'd replace "downsizing" with "lightweighting" and "fitting more efficient engines" that use gasoline direct injection and turbochargers. As we've said, we won't all be driving golf carts under the new standards.

As we've also noted, the market may drive a shift to smaller, more efficient vehicles if gasoline prices rise--or other incentives in a US government energy policy raise the price of fuel. But that shoe hasn't dropped yet.

Knittel's full 53-page report, Automobiles on Steroids: Product Attribute Trade-Offs and Technological Progress in the Automobile Sector, can be downloaded as a PDF at the link below.

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[UC-Davis Institute of Transportation Studies; hat tip to Geoff Archer]