For all the heat they take from critics, automakers have done a remarkable job increasing the efficiency of automotive drivetrains. Technological advancements like turbocharging, direct injection, variable timing and dual-clutch transmissions with seven or more gears have helped squeeze more miles from every gallon of gas.

There’s just one problem: These engines are powering ever heavier, ever more powerful automobiles. As a result, the technological advancements of the past 30 years have brought only small gains in real-world fuel economy, says MIT economist Christopher Knittel.

This would seem to go without saying, as anyone who’s driven, say, the 556-horsepower Cadillac CTS-V can attest. But Knittel’s quantified the problem. His research found the average fuel economy of automobiles sold in the United States rose a little more than 15 percent between 1980 and 2006. But the average curb weight of those vehicles climbed 26 percent and horsepower jumped 107 percent in the same time.

“Most of that technological progress has gone into [compensating for] weight and horsepower,” Knittel said in a statement.

All factors being equal, fuel economy increased 60 percent in that time, Knittel notes in “Automobiles on Steroids,” published in American Economic Review. If we were driving cars that featured today’s drivetrain tech but had the weight and power of vehicles in the Reagan era, the average fuel economy of the American fleet would have climbed from 23 mpg in 1980 to roughly 37 mpg today. That’s 10 mpg better than the current average, and 1.5 mpg better than the goal the Obama administration has set for 2016.

These are not insignificant figures, given that the transportation sector accounts for 30 percent of the country’s greenhouse gas emissions.

Knittel reviewed data from the National Highway Transportation Safety Administration, auto manufacturers and trade journals. His research found that light trucks (which include SUVs) represented about 20 percent of passenger vehicles sold in the United States in 1980. That figure had climbed to 51 percent by 2004.

“I find little fault with the auto manufacturers, because there has been no incentive to put technologies into overall fuel economy,” Knittel says. “Firms are going to give consumers what they want, and if gas prices are low, consumers are going to want big, fast cars.”

Between 1980 and 2004, gas prices dropped by 30 percent when adjusted for inflation. Knittel makes that point before stepping firmly onto the third rail of transportation policy by suggesting the best way to curb automotive emissions is to raise the gas tax.

“When it comes to climate change, leaving the market alone isn’t going to lead to the efficient outcome,” Knittel says. “The right starting point is a gas tax.”

Before you prepare to burn the man in effigy, you might do well to recall that the federal gas tax stands at 18.4 cents per gallon and has not been raised since 1993. (It also is not indexed for inflation.) As of January 2011, all taxes on gasoline averaged 48.1 cents per gallon.

The only way to foster true change, Knittel argues, is to change our policies. It is incumbent upon the government to “create a structure that leads to these technologies being put toward fuel economy,” Knittel says.

To that end, the Obama administration has raised the Corporate Average Fuel Economy of cars and trucks to 35.5 mpg by 2016 and 54.5 mpg by 2025. Knittel says automakers could meet those benchmarks by simply maintaining the rate of technological innovation seen since 1980 while also reducing by 25 percent the weight and horsepower of the average vehicle.

That’s an important point, as it challenges the industry’s well-worn argument that meeting tougher fuel economy standards poses a technological challenge.

“For a long time automakers argued that they did not have the technology to boost fuel economy, but they kept proving themselves wrong by putting it in our cars year after year, just for performance and weight instead of fuel economy,” said David Friedman, deputy director of the clean vehicles program at the Union of Concerned Scientists. “As a result, a family car today delivers the performance of a muscle car from the 60s.”

Now, Friedman says, it is time for automakers to shift their focus to making vehicles more efficient for the sake of overall efficiency, not performance. We can do so, he says, without sacrificing weight or performance.

“For the past 25 years, fuel efficiency technology has gone into boosting performance and vehicle size,” he says. “Now we need to put at least the next 25 years of fuel efficiency technology into saving consumers money at the pump through higher fuel economy.”

We’re already seeing this, as automakers pare weight from their vehicles and downsize the engines they’re putting in them. Six-cylinders are replacing eights, while fours are replacing sixes, without severely impacting performance. We’re even seeing wee little three-cylinder engines that perform like larger four-bangers.

Knittel goes even further, saying we could see a fleet-wide average of 52 mpg within eight years if automakers built cars of the same weight and power we saw in 1980. That, however, does not seem likely in a nation so fat that Fiat had to revamp the interior of the diminutive Fiat 500 so we might actually fit inside. We, as a nation, also seem utterly convinced that tiny cars are deathtraps and we absolutely need at least 300 horsepower for running to CostCo.

Still, there are some in the auto industry picking up what Knittel is laying down. Legendary Formula 1 designer Gordan Murray, for example, has long argued that radically cutting the size and weight of our cars is the fastest, easiest and cheapest way to reduce fuel consumption and CO2 emissions. The innovative manufacturing method he’s devised to do just that is getting attention from automakers.

Yet Knittel remains skeptical that tighter CAFE regulations will, by themselves, have the same impact as a tax hike. Raising the fuel economy rules will actually make it cheaper to drive, which means people will drive even more — and therefore consume more fuel. If we truly want to put a dent in greenhouse gas emissions, he argues, raising the gas tax is the way to go. Only then will consumers demand more efficient cars and use less fuel.

Photo: matt-hurst/Flickr