there has been virtually no improvement in fuel economy.
See it yourself.
Source is The Spritmonitor, a website where drivers, mostly from German-speaking countries, share observed, real fuel economy of cars they own and drive on daily basis.
Here is the outlook for diesel cars:
And here goes for petrol ones:
And that's how the matters look for a selected case - a diesel powered Audi midsize family car...
...with a for cylinder engine (six cylinder diesels have been excluded):
Why is the improvement so pitiful or sometimes even there isn't any?
Two main reasons stand for this.
First off, any improvements in engines combustion process has been cannibalized by customers constantly raising requirements for cars to offer more than just travelling capability. Additional safety and comfort and space inside (translating to larger, heavier bodies) all add weight. Modern cars have to carry reinforced body structures, systems for controlling braking, power and damping distribution, airbags, seatbelt pretensioners, safety padding as well as automatic gearboxes, better sound insulation, climate controls, sound systems and electric assistance in just everything, even activities as rare as adjusting seats and mirrors. Moreover, customers demands cars accelerating faster and faster, despite living in a world of speed limits. This pushes engines sizes and weights up. Poshness also makes things worse. Wider tires and larger car body (SUV fashion) increase tire rolling resistance and drag, respectively, when in motion.
But there's more essential reason - upper limit of efficiency an internal combustion engine (ICE) can reach, imposed by limitation of Carnot cycle, an idealizing model of how heat engines work. The only part of combustion energy an ICE can put to good use is when mixture of fuel and oxygen increases its volume when burning and therefore turning into exhaust gas. Increased pressure pushes pistons down what is then converted into rotating motion. And that's it. Except of cases when very small fraction of heat energy warms the car cabin, all the heat (some of pressure too, due to kinetic energy of exhaust gases) is wasted.
Put all that together and what you have is virtually no gain in fuel economy over 40 years.
Some may argue that small number of old cars may not represent overall average consumption well. That can be true for very low figures (say, less than 10), but random nature of selection increases the chance to produce correct simulation result, particularly when the sample becomes large. Such one would also at least partially represent market breakdown in terms of cars sizes and, therefore, fuel economy classes. Computing average fuel consumption very precisely is also not really the purpose. It's rather a simulation of where we are in terms of improving fuel economy and how we performed over the years. The estimates may be wrong (and to little extent, they certainly are) but a giant leap in the matter would have been spotted already when investigating just a dozen of randomly chosen cars. Nothing like this was observed. Notable is that for a selected case of a popular vehicle, Audi mid-size diesel family car, where chances of miscalculation drop dramatically, generic trend was the same as for the entire bulk. This supports credibility of the overall observation.
Others may suggest that, in fact, there's been great increase in fuel economy over time since modern cars are much more powerful than the ones in the past and yet, they use similar amount of fuel. While this statement is true, it's necessary to realize that for most of its usage, a car either moves at a constant pace on highways/motorways or calmly accelerates in dense traffic, limited by other vehicles on the road. Foot hard down to use all the power is a rare event. This was true years ago and is now and it's the activity performed most of the time what has largest share in the amount of fuel consumed. A car now can have 150HP while its predecessor had 80, still only, say, 40HP will be used for 95% of the ride.
Sources
Data have been accessed on 24.09.2008. When accessed now, results may vary since Spritmonitor users constantly add fuel consumption data and userbase changes also. Still though, changes are expected to be so little they can be considered insignificant.
Diesel cars:
1960-1970
1971-1980
1981-1985
1986-1990
1991-1995
1996-2000
2001-2005
2005-2008
Petrol cars:
1960-1970
1971-1980
1981-1985
1986-1990
1991-1995
1996-2000
2005-2008
2005-2008
Audi midsize diesel family car:
Audi 80 B2 1978-1986
Audi 80 B3 1987-1991
Audi 80 B4 1992-1995
Audi A4 B5 1995-2001
Audi A4 B6 2002-2005
Audi A4 B7 2006-2007
Audi A4 B8 2008
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