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Looking Back, Projecting Forward: Light Duty Vehicle Efficiency in the US

The PSFI of An and DeCicco found a linear progression in efficiency improvement over the past three decades. Click to enlarge. Data: An and DeCicco

In the increasing debate over technology strategies and public policies to mitigate greenhouse gas emissions and begin reducing petroleum dependency, it's useful to analyze past trends in vehicle efficiency as well as to try to assess the potential impact of powertrain choices for the future.

Two papers presented at the SAE World Congress addressed those topics. Feng An from Energy and Transportation Technologies, LLC and John DeCicco from Environmental Defense presented an analysis of past trends in technical efficiency for the US light-duty vehicle fleet, including a few recommendations for the future. Emmanuel Kasseris and John Heywood of MIT’s Sloan Auto Lab assessed the potential improvement of automotive powertrain technologies 25 years into the future.

Using fuel economy alone as a measure misses trade-offs between fuel economy and other attributes of light vehicles that represent technical efficiency gains. An and DeCicco combined fuel economy with performance and size to create a “Performance-Size-Fuel economy Index” (PSFI) for quantifying technical efficiency.

Applying the PSFI to vehicle data since 1977, they found generally stronger efficiency improvement rates over the past 25 years than prior analyses had determined. Their results also suggest a diminishing growth rate for fuel economy at higher absolute levels of size and performance.

They found that relative to a 1977 baseline, technical efficiency as measured by PSFI increased an average 5.3% per year for cars and 3.1% per year for trucks.

For cars, they defined PSFI as the product of EPA’s sales-weighted horsepower-to-weight ration and sales-weighted “cubic feet mpg” statistics. For trucks, they defined PSFI as the product of EPA’s sales-weighted horsepower-to-weight ratio, wheelbase in inches, and mpg statistics.

In analyzing the results, they determined that distinct phases of technical efficiency gains have occurred over the past 30 years. These phases are:

  1. Accelerated fuel economy growth (1977-82 for cars, 1979-81 for trucks). With the implementation of CAFE, fuel economy trends outpaced the PSFI trend, and were accompanied by reduction in performance and size.

    This phenomenon was most obvious for light trucks, where the power-to-weight ratio was reduced by as much as 13% by the early 1980s to allow rapid fuel economy gains.

  2. Limited fuel economy growth (1982-88 for cars, 1981-87 for light trucks). Fuel economy increased during this phase, but at a slower rate.

  3. Exclusive performance and size growth. (1988-2005 for cars, 1995-2003 for light trucks.) Fuel economy remained flat, with all technology advances going into performance and/or size increases.

  4. Accelerated performance growth (1987-95 for light trucks). Fuel economy decreased while performance and size increased.

  5. Exclusive fuel economy growth. This situation, in which all technology advancement is used to implement fuel economy, without sacrifice of enhancement of power and size, “does not appear to have occurred for any appreciable time to date.

Although PSFI has increased steadily over the past two decades (about 50% for cars and 30% for trucks), it is almost completely due to performance gain. Thus, the way technology advances largely were applied over the past twenty years may have hurt the potential for future fuel economy improvement, assuming that feasible rates are linked to an underlying technical efficiency trend...and that market conditions do not shift towards smaller vehicle size or lower performance.

The implication is that either an accelerated rate of technological progress, or a turning back of the clock on performance and size levels, might be needed to improve fuel economy at rates comparable to what was achieved in earlier years.

Kasseris and Heywood modeled out the potential improvements in fuel consumption and performance of a variety of powertrain technologies with a focus on the fuel consumption and GHG emissions of the US light duty fleet in 2030.

They conducted their analysis from an overall vehicle system point of view (including such aspects as improvements in aerodynamics, transmissions and tire rolling friction), and with an eye on the total fleet impact, not just on new vehicles.

For a significantly different powertrain technology to make a difference in fleet petroleum demand and GHG emissions it must first penetrate the fleet in significant numbers. Before penetrating the fleet in significant numbers, the new technology must become a significant fraction of all vehicles manufactured. For significant production to start, the new technology has to at least be close in market competitiveness with existing technologies.

Most of the new propulsion technologies now under development (such as fuel cell vehicles, electric vehicles, or plug-in hybrids), still have technical challenges to solve before they can be considered market competitive. Although new technologies could have an impact at the fleet level in the mid- to longer term future, this is less likely in the short term.

Projected possible improvements in fuel consumption for a lower-performance sedan with a variety of future powertrains. Click to enlarge. Data: Kasseris and Heywood.

For those reasons, the researchers limited the study to analyzing internal combustion engines using petroleum-based fuels or blended biofuels. They assess the powertrain technologies on the basis of three vehicle platforms: a lower-performance mid-size sedan, a higher-performance version of the mid-size sedan, and a representative pickup truck. They chose the Camry as the model for the sedan, and the F-150 as the model for the pickup.

They held the size and the performance of the future vehicles constant at the level of 2005 models to compare fuel consumption of the different technologies on an equivalent basis. Vehicle simulations were performed using the ADVISOR software.

Among their conclusions were the following:

  • The current relative advantage of diesel over gasoline engines is likely to be reduced in the future.

  • Turbocharged gasoline engines have the potential to become almost equivalent with low emissions diesel engines. To realize that potential, however, the limitation of knock must be dealt with.

  • Hybrids maintain a comparative advantage in terms of combined fuel consumption in the future, which is likely to increase. “The main reason is that hybrids, being a relatively new technology, can benefit from several component (engines, motors, battery etc.) as well as integration/architecture improvements.

  • More efficient continuously variable transmissions could be a key enabling technology in improving fuel economy for hybrids and non-hybrids.

  • The relative fuel consumption benefits of diesel, turbocharged and hybrid powertrains are more pronounced for higher performing vehicles. “This is because sizing the engine for higher performance results in lower average engine loads.”

  • Future hybrids maintain a comparative advantage even for highway and aggressive driving patterns. The relative benefit remains the highest for urban driving, however.

  • Future gasoline engines will close the benefit gap with diesels in both urban and highway driving.

Fuel consumption results obviously varied with technology choices and drive cycles, but the study found decreases in fuel consumption ranging from 35% to more than 50% even on the more aggressive drive cycles.


  • Comparative Analysis of Automotive Powertrain Choices for the Next 25 Years (SAE 2007-01-1605)

  • Trends in Technical Efficiency Trade-Offs for the US Light Vehicle Fleet (SAE 2007-01-1325)



So cars have gotten about 150% "better" since 1977, but all of that goodness was put into performance, with virtually none into fuel economy.

Welp, that doesn't mean that we can't level off the horsepower game and start moving into fuel economy. I wonder: is it a direct tradeoff? If the performance inproves by 0%, how much will the mpg go up?


They forgot to track sales and production. At various points the market tanked becuase no one wanted to buy the puny junk.

Pwople can lie all they want about buying green... sales dont lie. Low perf small cars dont sell unless soo cheap most companies cant makr them.

Rafael Seidl

Dialing down performance, never mind safety and comfort levels, is something few consumers are willing to do voluntarily. It takes fairly massive market externalities, such as laws, taxes and subsidies/tax credits to counter the spiral of ever-increasing weight and power levels.

Only a sustained policy of raising the cost of owning and operating a powerful vehicle would generate sustained demand for high fuel economy - as the European experience shows, high fuel taxes do work but they are no panacea. This is one area in which public policy goals (energy security/global warming mitigation) will always be fundamentally at odds with consumers' mobility preferences, even in democracies.

Increased efficiency in the drivetrain, a key theme of this year's SAE World Congress, will continue to make no difference to vehicle fuel economy as long as increased passive crash safety and luxury featuritis neutralize all of the gains made.

Therefore, automotive electronics engineers need to stick their heads together and come up with globally standardized inter-vehicle data communications protocols (cp. Internet RFCs). After all, a differential GPS receiver plus short-range transmitter are considerably cheaper than a 360 degree radar/lidar device. Once cars can reliably maintain a dynamic map the traffic in their immediate vicinity and marry that with accurate terrain data, lightweight active safety systems can help reduce the amount of steel and the number of airbags required. This applies especially to serious incompatibility situations, such as an SUV crashing into the side of an econobox.

Crumple zones, rollover cages and seat belts will still be necessary but if present regulatory trends continue our grandchildren will be driving around in tanks by the end of the century. At least the spare tires and jacks are already gone from some vehicles, thanks to new technology.

On the luxury side, the focus needs to be on improving the electrical system (higher voltage, more efficient alternator, more efficient electric motors) and, on reducing the weight of interior components such as the seats and sound-proofing. The latter is especially hard to do.

Hopefully these trends will continue, as adding lightness is the best way to simultaneously improve both performance and fuel economy.


Let's stop the performance/weight gain scenario right now and start reversing it. Don't we have enough freaking size or power or should we reach for infinity, or at least the speed of light? People change. I did, while briefly flirting with this power trip during the eighties. But then, hopefully, one grows up and realizes the absurdity of all this power in traffic congested cities, highways, and even with speed limits up to 75mph.

Oh, yeh, and safety too. Note to drivers. We've engineered the cars enough to save you from a high percentage of accidents which would otherwise kill you . Now you are on your own. Hint. Drive more slowly and more carefully, and get a car with less power; speed and reckless driving will be more tempting.

If are streets and cities were one continous gunfight with people being murdered every day in every neighborhood regardless of how upscale, we would declare a national emergency and do something about it. That's what this is, one big gunfight, with the victims being the planet and, ultimately, ourselves. These vehicles are just loaded guns going off in all directions every day.


Some sensible transportation policies:

1. Progressively increase fuel taxes
2. Offset impact to low-income with income tax credit
3. Some $$ from #1 to improve public transit
4. Gradually reduce need to drive with better land-use planning combined with #3

Improve quality of life (and shrink waistlines) by building communities in which it is possible to walk, bicycle for most daily trips. Improving the existing system at the margins will only keep us running in place as population expands.


"PSFI increased an average 5.3% per year"
Compare this with the BS the CEO of GM was saying in an earlier article; 'a 4% per year increase in fuel efficiency would cost thousands of dollars.'

This proves they've already, in effect, done it. It was just hidden under the size and performance gains.

Rafael Seidl

Ai Vin -

PFSI is, roughly speaking, the product of MPG and vehicle size, see article. This is a very different measure than straight fuel economy, so you're comparing apples and oranges.


1 Every state ad city in the us should be forced on pain of finesand jail terms to improve there roadways such that a low performance put put car cab SAFTLY travel without any issues.

2 Every job in any city or state must come with it the road/mass transit ability to QUICKLY CHEAPLY SAFTLY commute ANY TIME ALL THE TIME.

3 any city or state with more car accidents hen an acce[tible standard should be frozen out on new homes and bissinesses until the problem is fixed.

If any city or stae fails ro comply in a set time limit.. the feds take over and FORCE IT TO.

Making the dayly grnd a bit less like grindhoue or mad max and a bit more like the sound of music just might do wonders on peoples "need" for incredibke hulks...



Your post makes total sense to me. Progressively higher fuel taxes would change behavior even before hitting people in the pocketbook. Just the knowledge that the price will only go up will change buying patterns immediately. That's why the auto lobby will fight very hard to prevent fuel tax increases. So how do we get more people behind the idea?



I don't know how you get more people behind the idea of progressively higher fuel taxes and energy-efficient land use planning. Perhaps you do it by example; perhaps an enlightened state government could experiment with such changes in policy to show the way. It would preferably be a state physically large enough that many drivers couldn't just cross state lines to avoid the higher taxes.

How do you get people to think long-term? Think about doing what's right for their children and grandchildren?


IF IF ...IF... You replace one type of tax with a fuel tax AND provide very low cost cars with 0 interest loans.. you just might avoid being shot in officw.


We've had higher gas taxes in Canada for quite some time now. I can tell you that attitudes will change. I test drove a V8 Grand Cherokee last weekend to replace my old mini-van. Feels nice to drive and I can easily afford it, but, seems wasteful and childish(can't believe I'm saying that, believe me !). I'm not towing horse trailers or big loads.
So, Can't get myself to buy it.
Gov't policy might be the only way to modify thinking, in the short term.
Not sure "WHEN" or "HOW" the U.S population will come to this conclusion ?


Not sure "WHEN" or "HOW" the U.S population will come to this conclusion ?
With gas already well over $3/gal in some parts of the country, I think they are already coming to the conclusion that paying a hundred bucks to fill up blows chunks.

There's too much focus on "size", and not enough focus on "weight". If you are willing to pay for it, you can lose weight without losing either size or safety. Crush zones work better when they are longer, not when they are heavier. A huge mass in, say, a brake caliper is not doing anything for safety. That said, some vehicles are simply too large for what they do. The F150 would be a case in point.


In a market driven society it is what sells that matters. If powerful cars sell and advertising fuel economy does not, then they make powerful cars.

Ross Perot proposed a graduated $.05 per gallon tax every year back in 1992. That did not go over so well. I guess you could say that politics is market driven in a democratic system as well.


"advertising" is just another word for "brainwashing"
Powerful cars sell because they are advertised and fuel economy does not sell because it is not advertised.

Stan Peterson

I am trying to understand the graph presented. If I read it correctly, automobiles have inproved from 70 "units"in 1975 to 170 "units" in 2005 while for trucks, the figures are 60 to 120 "units" in the equivalent period.

This is an efficiency improvement of 2.8 times for autos and 2.0 times for trucks. That is truly a great achievment at the same times that auto quality has gone from 700+ initial defects per hundred for US and EU autos and 350 or so defects per hundred for Japanese auto brands. To where today the very best are around 100 defects per hundred for some Japanese brands (Toyota) and 112 per hundred for most domestic brands and 120 or so for European brands, essentially a barely measureable difference.

At the same time efficiency has gone up 2.8 times; quality has improved almost 7 times.

We know that fuel efficiencies are about ready to soar 3 or 4 times, with clean deisels, and HEVs, soon followed by PHEVs, within the next half decade. The "intelligent highway" or rather the decentrtalized intelligent car, the REAL 21st century Mass transit, is coming along as well.

Any serious observer of the ground transport scene cannot but be full of confidence in the future.


Help me understand something: it seems BMW saves 4% on the new 1 series by turning off the generator when not in use. It also saves 2% by using an electric water pump rather than belt driven. This is 6% fuel savings for the cost of (at most) $3-5 per auto.

Now take this one step further, and add start/stop a la Saturn (as BMW also did), which would not be very expensive (let's call it $500/unit) at million unit volumes.

So for $500/unit, we could cut consumption by 16%, with no real technical, financial or supply chain barriers. Now, add 20-25% fleet penetration of diesels and full hybrids, and we've certainly cut overall national passenger car and light truck fuel consumption by 20%. This could be done in the next five years, no?

What's stopping us here?

Am I missing anything?


When the price of oil changed dramatically in 1973 and OPEC announced its new dominance, the western world started seeking alternatives and substitutes.

At that time the best that could be done was to shrink the cars, and cut their power. This resulted in lots of unappetizing cars that were unsafe to drive, as they couldn't pass or merge satisfactorily. The domestic manufacturers had to rush through their downsizing, and it showed in under engineered cars, with poor reliability on newly down-sized parts. The domestics are still suffering from that 1970s lack of quality in the haste to downsize, forced by the government CAFÉ laws.

Eventually technological improvements resulted in more power from a general downsizing of engines but with advanced technology like all-alloy engines, VVT, SFI, and now DI, multiple valves, OHCs and computer control with microprocessors, produced more powerful, smaller, more economical, power plants. Today the domestic makers have a generational head-start in the most modern engines.

In addition, the NHTSA set standards for safety that resulted in improved safety in the downsized vehicles. But at the cost of more rugged components, heavier wrights, side beams, rollover standards, ABS, Stability Control and air-bags all over, in newer version of these down-sized platforms. Quality improved in second generation of down-sized parts, too.

The Horsepower wars were mostly fostered by the European imports seeking to differentiate themselves from domestic offerings; and to justify their astronomical prices.

That Horsepower war is now over. It really didn't have much more to run. Passing and merging can be done in a satisfactory manner with 6-8 second 0-60 mph performance. Who really cares about 0-60 in 3 seconds, except boy racers?

Its really unusable on the street. Cars can easily downsize the output of the new smaller engines, and they will, to increase fuel economy. So mileage improvements will come very fast, IMHO.

Eventually, both optimal and maximum, emissions and fuel economy with petroleum based ICEs will be reached. When HCCI is introduced, there will be little further improvements to be had. But thankfully a full substitute for Petroleum is, for the first time, becoming feasible.

You cannot remove dependence on Petroleum as long as your solution merely minimizes petroleum use. So HEVs like Toyota offers, are but a temporary partial step. You are still dependent on Petroleum.

But Electrified Ground Transport, can remove any need for petroleum fuel, in the last growing market for Petroleum which is Ground Transport. Despite the cater- wauling of Government and Green types, the "invisible hand" of market competition has been at work, seeking economical substitutes, and has driven these technological improvements and substitution.

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