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The Case of the Missing Gas Mileage

Despite automakers’ progress in fuel efficiency, vehicles’ mileage have barely increased

Published: Monday, January 9, 2012 - 12:02

Contrary to common perception, the major automakers have produced large increases in fuel efficiency through better technology in recent decades. There’s just one catch: All those advances have barely increased the mileage per gallon that autos actually achieve on the road.

Sound perplexing? This situation is the result of a trend newly quantified by MIT economist Christopher Knittel: Because automobiles are bigger and more powerful than they were three decades ago, major innovations in fuel efficiency have only produced minor gains in gas mileage.

Specifically, between 1980 and 2006, the average gas mileage of vehicles sold in the United States increased by slightly more than 15 percent—a relatively modest improvement. But during that time, Knittel has found, the average curb weight of those vehicles increased 26 percent, while their horsepower rose 107 percent. All factors being equal, fuel economy actually increased by 60 percent between 1980 and 2006, as Knittel shows in a new research paper, “Automobiles on Steroids,” just published in the American Economic Review, which is available as a pdf here.

If Americans today were driving cars of the same size and power that were typical in 1980, the country’s fleet of autos would have jumped from an average of about 23 miles per gallon (mpg) to roughly 37 mpg, well above the current average of around 27 mpg. “Most of that technological progress has gone into [compensating for] weight and horsepower,” Knittel says.


And considering that the transportation sector produces more than 30 percent of U.S. greenhouse gas emissions, turning that innovation into increased overall mileage would produce notable environmental benefits. For his part, Knittel thinks it is understandable that consumers would opt for large, powerful vehicles, and that the most logical way to reduce emissions is through an increased gas tax that leads consumers to value fuel efficiency more highly.

“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.”

Giving the people what they want

While auto-industry critics have long called for new types of vehicles, such as gas-electric hybrids, Knittel’s research underscores the many ways that conventional internal-combustion engines have improved.

Among other innovations, as Knittel notes, efficient fuel-injection systems have replaced carburetors; most vehicles now have multiple camshafts (which control the valves in an engine) rather than just one, allowing for a smoother flow of fuel, air, and exhaust in and out of engines; and variable-speed transmissions have let engines better regulate their revolutions per minute, saving fuel.

To be sure, the recent introduction of hybrids is also helping fleetwide fuel efficiency. Of the thousands of autos Knittel scrutinized, the most fuel-efficient was the 2000 Honda Insight, the first hybrid model to enter mass production, at more than 70 mpg. (The least fuel-efficient car sold in the United States that Knittel found was the 1990 Lamborghini Countach, a high-end sports car that averaged fewer than 9 mpg).

To conduct his study, Knittel drew upon data from the National Highway Transportation Safety Administration, auto manufacturers, and trade journals. That data show that a major reason fleetwide mileage has only slowly increased is because many Americans have chosen to buy bigger, less fuel-efficient vehicles. In 1980, light trucks represented about 20 percent of passenger vehicles sold in the United States. By 2004, light trucks—including SUVs—accounted for 51 percent of passenger-vehicle sales.

“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.” And between 1980 and 2004, gas prices dropped by 30 percent when adjusted for inflation.

The road ahead

Knittel’s research has impressed other scholars in the field of environmental economics. “I think this is a very convincing and important paper,” says Severin Borenstein, a professor at the Haas School of Business at the University of California at Berkeley. “The fact that cars have muscled up rather than become more efficient in the last three decades is known, but Chris has done the most credible job of measuring that tradeoff. This paper should get a lot of attention when policymakers are thinking about what is achievable in improved automobile fuel economy.”

Indeed, Knittel asserts, given consumer preferences in autos, larger changes in fleetwide gas mileage will occur only when policies change, too. “It’s the policymakers’ responsibility to create a structure that leads to these technologies being put toward fuel economy,” he says.

Among environmental policy analysts, the notion of a surcharge on fuel is widely supported. “I think 98 percent of economists would say that we need higher gas taxes,” Knittel says.

Instead, the major policy advance in this area occurring under the current administration has been a mandated rise in the Corporate Average Fuel Economy (CAFE) of cars and trucks. In July 2011, President Obama announced new standards calling for a fleetwide average of 35.5 mpg by 2016, and 54.5 mpg by 2025.

According to Knittel’s calculations, the automakers could meet the new CAFE standards by simply maintaining the rate of technological innovation experienced since 1980 while reducing the weight and horsepower of the average vehicle sold by 25 percent. Alternately, Knittel notes, a shift back to the average weight and power seen in 1980, along with a continuation of the trend toward greater fuel efficiency, would lead to a fleetwide average of 52 mpg by 2020.

That said, Knittel is skeptical that CAFE standards by themselves will have the impact a new gas tax would. Such mileage regulations “end up reducing the cost of driving,” says Knittel. “If you force people to buy more fuel-efficient cars through CAFE standards, you actually get what’s called ‘rebound,’ and they drive more than they would have.” A gas tax, he believes, would create demand for more fuel-efficient cars without as much rebound, the phenomenon through which greater efficiency leads to potentially greater consumption.

Fuel efficiency has come a long way in recent decades. But when it comes to getting those advances to have an impact out on the road, there is still a long way to go.

Article by Peter Dizikes. Reprinted with permission by MITnews.


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MIT News is the Massachusetts Institute of Technology’s central hub for news about MIT research, initiatives, and events. It reports MIT news directly and works with journalists around the world to help showcase the achievements of its students, faculty, and staff.


Response to Quality Digest or Fox News


Regarding your comment – it is somewhat naive and shortsighted. If you really analyzed my comment  and looked at the references you would see that I take actual data (not manipulated to show what I want it to show) and demonstrate that the liberal elites are chasing a symptom and not a cause!

Please note:

Consensus via Intelligencia is not proof nor is their recommended course of action the most prudent or advisable.

Quality Digest or Fox News

Looking at the anti-science comments, I wonder if this is Quality Digest or Fox News Website.

Concerning the anti-science thing

If you look at my comments you will see that I use basic science and statistics to demonstrate that CO2 is not causing global warming!  The global warming alarmists are the ones making anti-science statements.

More on CO2 as a pollutant and cause for global warming

Another thing to consider about combustion engines and emissions:

Consider the combustion equation – 2C6H6 + 15O2 > 12CO2 + 6H2O.

You will notice that for 12 CO2’s there are 6 H2O, there is a bias that exists among scientists and alarmists that blinds them to the fact that water (H20) is more of a greenhouse gas than CO2.  In fact H2O absorbs about 5 times more Infrared radiation than CO2.  It is irresponsible to concentrate only on CO2 and state that CO2 is a pollutant which again all life on this planet depends on!  Also, consider this equation:

CO2 (in water) > CO2 (in air) - This equation is driven to the right as the temperature of water increases.

(An example would be a cold soda going flat as it gets to room temperature).  It has been determined that the concentration of CO2 increases about 25ppm per 1.0 deg C increase in temperature.

Based upon this simple concept and to put it in quality terms an increase in CO2 concentrations is a symptom of global warming and not a cause!

This is why I state:

CO2 is not a pollutant and is not causing global warming (the data really does not demonstrate this - just run a regression analysis and you will see).  We need to stop this nonsense and develop our energy resources so we establish energy independence with renewable energy sources which include Coal and fossil fuels.  What we really need to control are the byproducts of fossil fuel combustion such as SO4, NO3 and the heavy metals such as Hg.  Our main concern should be carbon monoxide not CO2.  H2 would be the best energy source to exploit at this time.  This is what our discussion should be centered upon - which is the best energy source to set up in the short term realizing what are the hazardous by products of its use!

Isn't this America?

Someone please inform Mr. Knittel that the US was founded as a free country with a free-market economy.  Apparently he disagrees with the notion that consumers should be free to choose what vehicles they buy and producers free to fulfill their customer’s wants and needs.  Instead, he promotes a government-controlled automotive industry such as those found in socialist and communist countries.  The ease with which the intelligentsia and politicians promote socialist/communist policy these days is shocking.  I fear that many in the US have never known or have forgotten the atrocities that such governmental control can eventually lead to.

No, I don’t need the government to coerce me into buying a vehicle I don’t want or like just to pay homage to a dubious scientific theory.  I prefer to exercise life, liberty, and the pursuit of happiness – without Big Brother telling me what vehicle I can drive.

CO2 is not causing global warming


I would like to suggest we stop this nonsense about AGW/Global warming.  We could save ourselves a lot of money.

Think about it-CO2 is a molecule that all life on this planet depends to live and our EPA declares CO2 as a hazardous material based upon faulty data collection!

Please consider the following points:

Point 1) Measurement populations:

a)      Before going on to the answer, homogeneity of data brings to mind the current Climate Gate scandal. Global temperature trends over the past 150 years have been based on changing universes. 150 years ago, there were just a handful of weather stations, with very poor measurement systems, quite different to those today. The number of stations grew to 8000 in the 1980's, and then fell to around 4000. How can any valid conclusions be drawn about an "average”? This is a comment from a Donald J. Wheeler article about data analysis specifically populations.

b) Thinking about data analysis article – “First, Look at the Data” – Donald Wheeler. Combining this article with 1948 to 2008 Pacific Ocean temperatures you can see that there is a correlation between ocean temperature and the number of Hurricanes in the North Atlantic. What this correlation demonstrates is the Hurricane cycle runs in approx. 25 year patterns with no indication of a gradual increase of storms which counters the alarmist’s claims that storms will increase in severity over time due to global warming.

Point 2) "Only by playing with data can scientists come up with the infamous ‘hockey stick’ graph of global warming" By Ross McKitrick

Steve and I showed that the mathematics behind the Mann Hockey Stick was badly flawed, such that its shape was determined by suspect bristlecone tree ring data. Controversies quickly piled up: Two expert panels involving the U.S. National Academy of Sciences were asked to investigate, the U.S. Congress held a hearing, and the media followed the story around the world.

The expert reports upheld all of our criticisms of the Mann Hockey Stick, both of the mathematics and of its reliance on flawed bristlecone pine data. One of the panels, however, argued that while the Mann Hockey Stick itself was flawed, a series of other studies published since 1998 had similar shapes, thus providing support for the view that the late 20th century is unusually warm. The IPCC also made this argument in its 2007 report. But the second expert panel, led by statistician Edward Wegman, pointed out that the other studies are not independent. They are written by the same small circle of authors, only the names are in different orders, and they reuse the same few data climate proxy series over and over.

Point 3) Water (H2O) has special properties like sublimation that can explain why the icecaps and glaciers are melting. It is the Sun that is causing the warming (in cycles) and an increase in CO2 is a symptom of the warming and not the cause of it!

Point 4) my comments to the US Chamber of Commerce - Facing Our Energy Realities:

“The Wall Street JournalChamber, Others Support Legislation That Limits EPAA report on the advancement of a bill that is aimed at preventing the EPA from using the Clean Air Act to regulate greenhouse gas emissions. The Journal notes that the Chamber and other business trade groups praised the legislation.”

CO2 is not a pollutant and is not causing global warming (the data really does not demonstrate this - just run a regression analysis and you will see).  We need to stop this nonsense and develop our energy resources so we establish energy independence with renewable energy sources which include Coal and fossil fuels.  What we really need to control are the byproducts of fossil fuel combustion such as SO4, NO3 and the heavy metals such as Hg.  Our main concern should be carbon monoxide not CO2.  H2 would be the best energy source to exploit at this time.  This is what our discussion should be centered upon: “which is the best energy source to set up in the short term realizing what are the hazardous by products of its use!”

Another comment from someone else:

I find it extremely troubling that the Chamber of Commerce seems to believe in the false premise of anthropogenic global warming/climate change, or whatever it is they wish to call it today. It is especially worrisome when a supposedly conservative organization goes along with what we know is probably the largest fraud in history. Yet somehow they see a reason to give it credence and think that steps should be made to address this 'problem'. There is no definitive evidence to support any such claims that our planet is warming, in fact, there has been no significant warming for at least the past several years. The unfortunate part is that they want to say that science has everything to do with consensus, but real science is not based off of consensus, and not on some bizarre theory put forth to stifle economic growth. I would suggest that the Chamber of Commerce take heed of these facts instead of taking a politically safe position of moderation.

Please let's concentrate on what is important:

Becoming Energy Independent through all our energy resources!

Social Responsibility (SR):

The following is from an article titled – “Ford’s Focus – The automotive pioneer’s ideas on business practices, SR are still applicable today”. Quality Progress magazine, Sept. 2011 pp 48-52

This article is applicable to our energy situation -what I am trying to get at in point 4 above- and to spreading democracy globally in relation to the social responsibility of our leaders to us their citizens or customers! If our leaders-as well as us-applied the following as way of life we could sustain ourselves for a long time.