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Kris De Decker


Ahum, thanks to joaquintides at reddit who immediately noted that a decrease of 400 percent is impossible. A fourfold decrease (what I meant to say) = 75 percent.




My car doesn't run on kilowatts, though.

And how is it that cars, according to the EPA rated numbers, are MORE efficient at highway speeds than 'city' speeds (e.g., 17 city/24 highway) except of course, hybrids, which are a NEW design technology that makes them more efficient when in stop and go traffic? In that model, it would be more efficient for cars to never go under 55mph!

Granted, you talk a lot about trains in your article, and yes, on an unobstructed track, traveling at a steady speed, a vehicle can have more efficiency than anywhere else.

If the automobile industry ("The Big Three") had not buried "Smokey's Engine", automobiles would run at greater than 75% efficiency! And what other technologies has their collusion with "Big Oil" hidden from the innocent public?

Kris De Decker


I think your car is more efficient on the highway because it cruises at a constant speed, without much braking and accelerating.

That said, it is true that our vehicles are optimized for higher speeds than those I am aiming for, and thus might not give the fuel savings I am calculating here.

So in order to achieve the full 75 percent, we need a new generation of cars (no new technology, just a downsized version of what already exists). With cars this is easier to achieve than with ships or planes, which have a much longer service life.

I am not against hybrids, but they should be combined with lower speeds. And yes, electric cars are more efficient, but then you have to lower speeds anyway otherwise your autonomy will be very limited.

Tim Rosencrans


Dumb ideas seem to be flourishing these days. Transportation systems do not exist of only cars. Cars need roads. If all cars go at half speed then traffic is doubled. This is simply not doable in most areas.

Exactly how efficient is a car stuck in a traffic jam?

Kris De Decker


Maybe if cars go only 60km/h people would realise that on many occasions a bike or a train is a better alternative.

Maybe we can bring back the speed of cars to 30 km/h and keep the trains running at 180 km/h...



With everyone rushing to build windmills and solar farms, does it really matter whether we use more or less electricity to make a faster train? Perhaps the better solution is to ensure that at least 75% of the power comes from a clean source. I follow you for other fossil-fuel powered devices, but even that is simply a matter of time before we can find an alternative that makes the argument largely moot.

However, take your time is money discussion a bit further, and it becomes a relatively direct economics case study - a complex one, but it definitely becomes about utility vs cost. I don't think that people necessarily view faster things as having no ecological value (positive or negative) as much as this: they do not view the ecological negative as so much incrementally worse that it's more valuable than their time. For example, I like spending time with my family more than I like spending 1-3 hours commuting. From an environmental perspective, I would gladly accept any mode of transportation that would cut my commute time by 75% even if it was powered by coal, diesel, baby seals and virgin rainforest timber. I might think twice if it meant increasing the cost of my commute by 75% as well. This is the same decision some folks have made about transitioning to public transit or a bicycle in the world of expensive fuel - what is my personal cost threshold for the convenience of speed? Yes, environmental impact may help to break a tie between two things that are so close that they may as well be equal, but I don't believe that it's the primary decision factor in most people's lives.

Your argument about fast trains vs slow trains v planes is simplistic, because if there are multiple transportation alternatives, people will choose the fastest one within their means. The only way to ensure that people always choose the most ecologically friendly method of transportation for their person or goods is to put a price on each method based on its environmental impact. This may be different for each person and each situation, since it might be better for me to drive my car 2 miles to the store instead of needing a bus route that only services me and my neighbor, since my car is more fuel efficient at that speed and capacity. So unless all transportation pricing is set by a central authority with a big supercomputer that can run the numbers on each trip for each person, rather than oh, say, by capitalism, this is one of those things that we're going to have to run based on least common denominator. That is, make transportation as efficient as possible at the speeds and capacities necessary to make it attractive to the largest number of people that it can support and remain more efficient than the alternatives.

Kris De Decker


You make some interesting points, but in your conclusion you state that we need to provide a transport system "at the speeds and capacities necessary to make it attractive to the largest number of people that it can support and remain more efficient than the alternatives."

Could you put an exact speed limit on that?

Because if I look around me, cars and motorcycles are getting faster and faster. The speed limits don't change, but the machines and therefore also the drivers do.

This "necessary" speed you are talking about keeps climbing all the time, so where do you set the limit? If you don't do that, fuel consumption will keep growing, regardless of all energy efficient technology.

The guys you see on those pictures in the article, were racing at speeds that we now consider normal.



That's really my point - there is no way to set a speed limit in the name of fuel efficiency. I'm not advocating pushing speeds beyond diminishing returns - the market cost of fuel and technology imposes that limit already, hence the reason WHY we have no commercial supersonic jet. However, technology keeps moving the needle on the diminishing return point, and there's no reason not to take advantage of it. In your article, you even said that the faster train was more efficient than the slower train. As technology evolves, we come up with ways to make things go faster with less fuel used than the simple square of the velocity rule. Until we run up against the point where no amount of technology will make a commesurate increase in speed more efficient, there's no benefit to reducing speed beyond what is cost-effective.

George Seldes


Actually, when you factor human behavior in, power consumed DOES follow the cube of speed, with no need to reduce for the reduced time because distances AREN'T constant, at least not in N. America --- by which I mean that the first thing a North American does when offered a faster travel option is compute how much further he/she can live from where they routinely travel. So rather than at least compensating for the higher energy consumption rates by keeping the distance travelled the same, the Homo Non-Sapiens North Americanus will sprawl further and further out, as if impelled to keep total travel time at a certain level.



Some interesting comments here, too:




While I understand the arguments made here, would you please get your facts straight - how many cars can you think of that require 240KW (321 horsepower) to reach 160km/h (100mph)??? My car is an average size saloon car and has only 92KW (123 horsepower) and it is well capable of cruising at over 200km/h (112mph)!

Kris De Decker


Yes, David, you are right. I should have taken a lower value to make the comparison more realistic. But that does not make the comparison wrong.




Sure it would take twice as long on the road for a trip of a given distance, but who wants to take twice as long traveling. People would move closer to work.

You should retract you statement about dumb ideas. This site is filled with innovative and interesting ideas. Some of them may be more like thought experiments than actually being practicable, but it is still a very valuable resource.



Slower is not always better - much depends on gearing, weight, Cd, rolling resistance etc.

I agree with you that today's cars are far too heavy and inefficient. I drive a car that came out in 1999. The Audi A2 1.2 TDI that can run on bio diesel. It produces 81g/km CO2 on dino fuel and can return 3L/100km even with 4 adults on board with luggage! But look at Audi now! They stopped making the A2 in 2005 and now make monsters like the Q7 SUV.

Their lightest car is the overweight A3.

As for the Prius - well more needs to be written about that Toyota marketing ploy. Not bad around town but on the highway the MPG is not as good as a std diesel car.



Want to cut your commute time to about one minute each way, regardless of distance?


Kitchen table to home office desk, log in, and you're there. I designed one of the bits that makes the telephones work. It's now in a PBX platform that is available worldwide. I'm not going to engage in crass self-promotion by mentioning brand names.

Know what? Methane has started bubbling up from under the Arctic sea. Anyone else here see the news? Know what that means? It means say ByeBye to the human race unless we turn the whole damn situation around pronto.

Oh, and as for automobiles with 200 - 300 horsepower. That's the kind of power you need for a fully loaded tractor trailer, or for one of those gigantic rotary snow plows that will clear a 12 foot (four meter) swath of snow that's six feet (two meters) deep on an airport runway.

The idea that anyone would have an engine like that in an automobile is so insane that if we can't kick that habit, we friggin' *deserve* to go extinct in thirty years.



The figure of 60kph was mentioned in the article as a proposed speed limit. If this was a recommendation for cross-country travel then it is decidedly not to be taken seriously. That is less than 40mph which would take us back to the average over the road speeds of the 1920s. This will not happen. Period. Even the 55mph limit of decades ago was widely flouted and served chiefly to enrich the coffers of speed-trap happy municipalities and radar detector manufacturers. The current interstate limits of around 70mph are considered reasonable by most people and are not exceeded to remotely the same degree as was the 55mph limit. Cutting speeds from 70 to under 40 would result in an entire nation of traffic scofflaws as virtually no one would obey such a ridiculously low limit however much fuel it would save. This would amount to a prohibition of timely cross-country travel. We all know how well Prohibition worked out. Such low limits would be wildly unpopular with 98% of the driving public and so would be promptly repealed in the unlikely event they were ever imposed in the first place.

Current technology can build vehicles that get much better mileage at 70 than current vehicles resulting in real gains because most of the driving public would be unlikely to go any faster than they already do. Even exoticars can only use their high speed potentials for very short distances and their tiny numbers on the road do not affect the overall picture anyway. In any case even if by some miracle passenger vehicles could be built that could get 70mpg at 120mph it is extrememly unlikely that interstate speed limits would be significantly increased because the roadways simply are not designed for much higher speeds. Most drivers aren't designed for it either.

Kris De Decker


If you manufacture cars with a maximum velocity of 60 kph, nobody would exceed the speed limit.

Dr. Claude Miller


If most of you Speed Nazis were to run around the block, breathing through a small straw, we wouldn't have to deal with you pansies anymore.



Hi Claude,

Calling them nazis isn't funny, are they in anyway racist, let alone anti semitic?

Roland Smith


Since 2007, the hour record for pedal powered land vehicles stands at 87 kph: http://www.ihpva.org/hpvarec3.htm#nom27

The absolute speed record (200 mtr flying start) stand at 130 kph: http://www.ihpva.org/hpvarec3.htm#nom01

The world speed record for the fastest aircraft still belongs to the X-15, 7272 kph (4519 mph): http://en.wikipedia.org/wiki/X-15

Robin 'Roblimo' Miller


I cut my commute 75% by moving from a house where my home office/studio was in a separate building to one where it's in an enclosed porch built onto the main structure.

We also gutted the new place (a small house trailer) and insulted hell out of it. Our electric bills are now ~$70/month Spring and Fall, ~$90/month during (hot Florida) summers with a/c running, and between those figures in the winter, with electric heat.

But in the spirit of this article, when I am cruising the neighborhood in the future I will ride my bicycle slower -- to save fuel, not because I'm getting old. :)

Stephen w


you could travel at 4000 mph and use virtually no energy - just transport a dollop of it from your starting point to your destination - see evacuated tube transport http://www.et3.com/



One problem not brough up is article is that existing vehicles are designed for fuel efficiency at 80 to 100 km/hour. Driving existing cars at 50km/hour, actually gives a worse fuel efficiency. The entire engine, gearing, drive train and wheels width and other things would have to be redesigned if you want 50km/hour to be the most fuel efficient speed.

Cars are desinged so that 80 to 100km/hour are the most fuel efficient other wise, like the article suggests, you would burn 5x as much fuel driving at 100km/hour than at 50km/hour. No one would buy a car that burns a good 5L per 100km at 50km/hour, but then burns 30L per 100km at 100km/hour. So instead the cars are made to burn about 10L per 100km/hour at nearly all speeds 100km/hour or lower.

That is the exact reason why hybrid cars work - even though they are still only powered by fuel (the battery is charged by fuel). The battery allows the car to operate at extremely high efficiency at lower speeds, while still allowing the car to reach 80 to 100km/hour. Obviously the hybrids by definition will be slower at highway speeds, as they have to convert fuel to electricity first (energy loss) as well as burning fuel directly.

I still think its a good idea to reduce speeds to 60km/hour. But it will never happen, because it would require all existing cars and trucks to be dropped immediatly, and it would have huge implications for the world in general. For example property prices and zoneing of land is done largly on the fact that the majority of the population will travel only around 1 hour to work. So if you decrease the top speeds to 60km/hour you will reduce significantly the area in which people will consider a 'city' or 'urban' area, forcing everyone to live even closer together.



"Moreover, very good aerodynamics is incompatible with high speeds. Formula 1 racing cars have the worst drag coefficients of all vehicles on wheels, because of their large spoilers and very wide tyres. At higher speeds, it becomes important to minimize lift at the expense of better aerodynamics so that the car is not catapulted into the air."

That's a lot of bunk. To go fast on your average grand prix circuit you have to be able to corner fast and most of the aerodynamics of a F1 car is designed to keep the car on the track (the downforce – downwards lift – generated exceeds the weight of the car). To go fast straight ahead (or on a banked track) you only need enough downforce to avoid taking off (compare an F1 and an Indy 500 car). Good aerodynamics is not the same as minimzing drag at all costs: it could be, as in the F1 case, to minimize drag for the required downforce.

(And even at bicycle speeds, aerodynamics makes a difference – ask Greg LeMond ;-)



Hi Folks, Interesting article. As I currently understand it for the ICE (internal combustion engine), the optimum speed is about 55.6mph (about 90kph). This is due to the compromise between vehicle weight, energy use and drag coefficients. At low speeds you encounter the main problem of engine run time and inertia - especially starting from standstill. At higher speeds the drag takes over. Thus one ends up with an average efficiency curve for the current vehicle fleet (mpg/energy/pollution) that has its maximum point at about 55mph (90kph). This varies drastically when looked at on a per vehicle basis - see for example this 'random' test: http://www.metrompg.com/posts/speed-vs-mpg.htm The 'Pontiac firefly' test cruising in top gear at 30mph might be good for fuel consumption, by it will play hell with you main bearings! Also, to implement the savings mentioned above, one would have to not just lower the ex-urban speed but also raise the intra urban speeds or stop these journeys altogether as many (i.e. driving down to the shops) are less than a few miles (I know this varies depending of where you live) but for an average EU city: Table 1 Average trip length distribution for the city of Antwerp Distance (km) Trips (percentage) less than 1 ... 7.68 1-2............ 15.21 2-3............ 12.97 3-4............ 9.69 4-6............ 14.73 6-8............ 10.69 8-10............ 6.00 10-15........... 6.17 15-20........... 1.33 Greater than 20... 15.53 From: C. Mensink,I. De Vlieger, & J. Nys, 2000, "An urban transport emission model for the Antwerp area" Published in: Atmospheric Environment 34 (2000) 4595}4602 Another point is that emissions of CO2 are in fact a better indicator of fuel used than energy. This is because the CO2 relates more directly to the fuel used than does the energy produced. For instance, at lower speeds proportionally more energy goes into running the engine and ancillary equipment than towards moving the vehicle - hence the problem of high pollution levels in traffic congestion with all those idling engines and stationary/very slow moving vehicles. Systemically reducing high speed on long journeys will have an impact, but for intra-urban driving/fuel use, it will have to be a case of abstinence - over 60% of urban journeys are under 6k! That’s under 3.75 miles!!! Best Wishes, Sid

Morten Lange


gurusid : It seems to me that some of your assumptions are false.
1. You talk about congestion as if that was a product of driving at low speeds. In fact it is the other way round. Lower speeds begets better throughput and more flow in city traffic.
2. You seem to imply that if at high speeds you do not need to accelerate as often. This skewed proposition is probably caused by people driving at really high speeds primarily on highways. There large amounts of money have been spent on an inefficient means of transport, not being paid for by the users. Additionally : close to urban centres and in many other places,if infrastructure and las allow speeds to be high, that increases the competitive advantage of cars over other more efficient modes like buses, trains, bicycles and walking, thus reducing the overall efficiency of the system.

It should be evident that it is the efficiency of the transportation system that matters, not the individual vehicle.
But of course the efficiency of individual vehicles are important factors. Especially when considering the effects of mode shifts to more efficient means of transport away from the car. A transportation system where the modal fraction (modal split) of public transport rises, or the modal fraction of bicycles rises will be improving energy efficiency, and generally reducing pollution, given that other things remain unchanged. But no matter what speed cars run at.

Stuart M.


This is a fun site! I loved reading about Citroen 2CVs and wood-burning cars. This article is of course completely right: there really is no reason for cars to go 70 mph. I live here in Japan. All the cars can go fast here too, but the national speedlimit for country roads is 60 kph (38 mph)! There are toll expressways where you can drive faster, but 60 kph is the predominant speedlimit. Of course, not everyone keeps to that limit, but if you're caught doing 80 kph (50 mph), you lose your drivers license. This means most people drive about 70 kph (44 mph) on the highways. I have a 4 wheel drive(!) Toyota Tercel that gets about 48 mpg when driven on the highway at these lower speeds. But the best is yet to come: my Toyota Tercel is 14 years old! I bet the lower speeds also make the cars last a lot longer and I see a lot of cars even older than mine driving around here.

The Japanese do something else that is very commendable: they have a class of cars which cannot have engines bigger than 660 cc's. These so-called K-cars are very popular because they are taxed way less than other cars (I have to fork over $1000 every two years for my Tercel). Most seat four comfortably and their performance, mainly because they are lighter, is about the same as normal cars. Over half of all cars now sold in Japan are K-cars.

Yes, when I first came to Japan, I had some serious withdrawal symptoms when faced with traffic that mosied along at 40 mph, but I got used to it. People forget that America was conquered at horse pace and those horses weren't galloping most of the time! Now whenever I go back to California for a visit, I drive 55 mph, but only because those big trucks are sitting on my bumper in the right lane.

Mike Black


I read these comments and see lots of reasons why we can't slow down, move closer to work/school/shopping, spend less on vehicles designed to sorta-survive at high speeds, etc .

Then I read another study talking about how oil is getting harder to find and more expensive to extract and I wonder exactly what it will take for people to start negotiating a new lifestyle.

Cool site BTW!

Frank Mancuso


The more fuel a motor can safely turn to heat the more efficient. Right? Wrong. A Top Fuel Dragster consumes 8 gallons of fuel in under 4 seconds to go 1000 ft. It's the speed that consumes the fuel. Over 300 MPH in just over 3 seconds.

K Cartier


When I mentioned this story to a friend of mine, his wife mentioned that during World War II, the national speed limit in the U.S.A. was 45 mph. He is 90, she is 87; they were there. This is an historic example of limiting speed limits to reduce gasoline use. She also said that another personal reason for limiting speed was that new tires were not available.

I'm wondering if there are any reports from the period that showed any relationship between automotive gasoline use or gas mileage prior to the imposition of the wartime speed limit, during it, and after it. If available, they might support your thesis with historical evidence.

Your article shows good reasoning. What does one DO with the time saved by getting there faster? That varies, but it wasn't used to enjoy the trip.



If we don't increase performance, it will be increasingly difficult to increase efficiencies. Only problem is that the motor car's development has been sales-led rather than engineer-led.

High average cruising speeds can be very economical, since up-inclines can be crested using kinetic energy. What ruins the fuel efficiency of most cars today is their excessive mass - it could be argued that since most of the world's roads are congested and require constant acceleration and deceleration, mass (or lack of it) is far more important than aerodynamic drag.

High speed was regarded as life-threatening in Victorian England - over 60mph and you would quite possibly die, it was said. Low speed has its place but so does high speed - why waste time counting down the markers on a motorway/autobahn/autoroute?

Sarah And


I came across this site by accident while looking for a more efficient charcoal BBQ!

What you say is very interesting & also the comments about the optimal speed. I guess that for a speed that is too low, nobody will keep to the speed.

For all intents & purposes, I think a vehicle that carries 1 or 2 person like an enclosed bike is very practical except the cost is still way too high at the moment.

Name Withheld


I'm laughing out loud at the awesome idea of slashing all of America's 55 mph speed limits down to 27.5 mph. I would love it. But sadly, I don't see this happening ANYWHERE. Not even in uber-eco places like Seattle. It'll never even make it to the discussion table. What does that say about human nature, about our society? Civilization never seems to willingly go backward. All of America could convert to an Amish lifestyle of family farms and horse-drawn buggies. But they won't. Not willingly. The US government could levy a $10 tax on every gallon of gasoline to curb carbon emissions. But they won't. The US government could ban all new housing construction in counties where there are existing vacant units. But they won't. The US government could require employers to allow telecommuting for all employees interested in doing so. But they won't. The US government could ban coal production. But they won't. The US government could enact a one-child policy. Or even eliminate the current child income tax deduction. But they won't.

Alex N


Civilization will never willingly go backward. The whole thing is based on a story of ever increasing advancement that people tell themselves - or, more insidiously, pick up from the "background noise" of our society. Like building a mosaic, kind of. Another story is man's rule over all - even if scientists say we are part of the natural world and observant laymen say we're killing ourselves doesn't mean that is the story that gets acted out.

The more one studies the more one finds odd things - hunter-gatherers used to live longer, healthier lives with a lot more free than city-dwellers, for example. Why, then, were they stigmatized as backwards barbarians? Because they were not taxable and could not be ruled. In the old times, power = concentrated manpower, and concentrated manpower required lots of calories. Cue densely packed, sedentary agriculture to which people usually were forced to. (I mean, worse health, lots of work, monotonous diet, risk of epidemic disease, taxes, forced labor, conscription, the devastation of your farm by a passing army whether friend or foe...)

The reason the barbarians were stigmatized was because people were literally running to the hills and other places where concentrated manpower, and hence the power of the state, could not be projected. They represented an opportunity for escape, and freedom.

Apart from that, agriculture has a nasty habit of turning ecosystems into desert and lots of hungry people, which directly causes civilization's ever-increasing drive to expand. In the past, terrain held it in check. Now, they can project their power basically anywhere, turning the cancer of civilization metastatic. Icky business.



Apart from stigma, hunter-gatherers also got to experience many diseases. If there were more agricultural people, that's probably because fewer of them starved to death, and the increased transmission of disease from living in larger groups must not have made up for the increased food supply. Which would you choose? Currently, lifespans depend a whole lot on public health measures. Places where public health measures are inadequate have to deal with lots and lots of diseases that we in the USA have almost forgotten, although a few are coming back in a small way due to the anti-vaxers. I for one am not pining for the "good old days" out in the forest. Of course we can now blow ourselves up, but it hasn't happened so far.
Stuart M:
Japanese people can probably tolerate the low speed limits because they live on an island (ok, islands) that is much smaller and more densely populated than the USA is. If not, even with the restrictions on engine power and such, I'm sure the speed limits would be higher and even low powered cars would be faster. Keep in mind that people don't just use cars for joy rides. They are generally trying to get somewhere specific. If you have one Walmart (ugh!) for every 20,000 people, that will be a much shorter drive in the Tokyo area than it is in rural Pennsylvania. (I have no idea what the usual ratio of Walmarts (UGH!) to population is, but the point still stands.) Of course, if the US zoning laws were revised, a lot of those stores could be much closer. Plus I'd like to point out that a popular uprising in response to lowere speed limits will not be very energy efficient.

If we really want to reduce people's environmental impact, we should create enough Manhattan's for all of us to live in. Big apartment buildings will use much less energy per capita to heat and cool, cars won't be practical, but subways will, local attractions will be numerous and close by, etc. Manahattan makes Boston look like Podunk, but the increased density meant I could walk to a whole lot of interesting music, art, bookstores, a couple of malls, and at least two supermarkets. However, I'm not volunteering to live in the city, since sprawl has pushed many of the open spaces I want to visit (and fly model airplanes at) too far out. Our current culture of zero tolerance and the pursuit of selfishness makes it worse, of course.



There are some misconceptions here about waterborne vessels:

-Really large cargo vessels operate at less than hull speed. Even the USS Enterprise, a nuclear powered aircraft carrier, topped out at only about 3/4 of hull speed. (assuming 1,000 foot waterline length. The overall length was above 1,100 feet) Generally, the accepted formula is that the hull speed in knots is 1.38 times the square root of the waterline length. But 1.38 implies more precision than we can have without knowing more about the hull.

-Ships are far more efficient per ton mile than anything else we have. Slow down a ship and you'll force people to use something that uses much more fuel. Or you'll have to build and break twice as many ships to carry the same amount of cargo, unless you're willing to accept an economy that's half the size. And in that case, you can probably forget about slowing down the ships.

-A long skinny hull can go well above nominal "hull speed" without that big rise in power. Even a human powered one. I am no athlete, but sprinting in a single scull, I was able to excede nominal hull speed, though not by much. People who sail Hobie Cats and any number of other skinny hulled boats know this is true. Other ways of evading hull speed are planing or hydrofoils, which are shown in the sailing and human powered craft you mention. But that's not really the same kind of vehicle at all. A planing or hydrofoil vessel is mostly out of the water.

I don't know if it was here, but I read someplace tonight that hydrofoils were dropped because fuel consumption was too high. I really doubt that. Quite the reverse. Say a 1,500 lb planing boat can go 40 mph on 100 hp. (That's very close to my real life experience.) We'll assume that the boat is about as aerodynamic as a large car, so the power to overcome air drag might be about 10 hp. If a hydrofoil can achieve an L/D of only 10:1, the prop and drivetrain together are about 60 percent efficient, and the air drag is similar, it will only require something like 37 horsepower.

-Any small water vessel going 511 kph is more of an aircraft than a boat, and is essentially airborne at speed. It would likely be safer and significantly faster if flown higher. Note the aerodynamic tail surfaces on the Spirit of Australia.
For a more intuitive demonstration of just how close to flying race boats are, check out this video.

Submarines are NOT slower than ships. Comparing a fast attack sub (which seems to be the kind of sub you're talking about), with the Spirit of Australia is like comparing a Ferrari with an RV which carries a crew of several, enough food for months, it's own air supply, and a bunch of heavy weaponry, while making almost no noise in the middle of it's speed range. The Spirit of Australia's "cargo" was one guy. Submarines that are sufficiently submerged don't have "hull speed", because they don't make surface waves. They can also go under bad weather instead of around. At least the nukes can. If nuclear power was cheap and unregulated, we might see nuke powered subs carrying cargo at maybe 40 or 50 knots. (I'm not saying this is a good idea.) Keep in mind that the speeds reported for fast attack subs are almost certainly understated, unless someone let the cat out of the bag. A sub is far better off in a tactical situation if the other sub doesn't know just how fast it can go.

If you want a really fast manned underwater vessel, take the bomb out of a superfast torpedo and install a sealed cabin for a person.

By the way, thanks for the info on the Schienenzeppelin. I had never heard of this ridiculous creation.

P.S. The human powered hydrofoil shown is not Decavitator. Decavitator was a later version so named because the hydrofoils were fully submerged and didn't have the "ventilation" problems associated with the surface piercing foils on the earlier version shown. Decavitator had a number of interesting but simple solutions to the problems that came up. However, I must advise anyone working with human powered hydrofoils to make sure the main "wing" goes on only right side up. Especially if you're going to test it after all night work sessions. (No, I was only a spectator that day.) ;-)

Andrew MacDonald


Thank you for the article, and the many others I have enjoyed here. I have long thought that the fixation with yet faster cars is pointless. However in Australia, with the vast distances, it would be inconceivable now to travel about at 60, or even 80kmh, on the open roads. I did it once, in an old Kombi, around Australia at an average of 8okmh. Never again!!

Martin Fierz


Kris, I love your site and I love your many interesting ideas, but this article is flawed. When travelling from A to B with any vehicle (car, train, ship, airplane, whatever), you find 4 things that consume your energy:

1) drag resistance which scales with medium density and speed squared
2) external friction, e.g. from the road; this will scale with the vehicle mass
3) Losses due to braking which will scale with vehicle mass, speed squared, and the number of stops per kilometer. This one is important in city traffic and the reason why hybrids might be interesting (but only in cities), and it's irrelevant for highway traffic.
4) internal friction e.g. from cylinders etc. which will scale with RPM. My feeling is this one is not very important but I'm not 100% sure.

It's not too hard to figure out the numbers for 1,2, and 3, and you will quickly see that halving the speed from reasonable values such as 100 km/h or 60 mph will definitely NOT reduce fuel consumption by 75%. It will only reduce 1) by 75%, but will do nothing for 2) which is also important, even at highway speeds. To do something about 2), you need also to reduce vehicle mass.

For ships, 1) dominates completely, and thus your conclusion is correct for ships.



The idea to reduce speed makes sense, but you don't analyse why people opt for speed. Driving or flying are acts during which people have limited options except to steer the vehicle or endure the trip. Traveling by train in contrast usually allows more or less other activities such as concentrated reading or even typing on a laptop. An autonomous transport vehicle could be quite slow if it was a comfortable office and such work becomes suitable for many people (writing code, texts or remote controlling robots). If our commute becomes part of our working hours the need for speed for this part of the day drops dramatically at a slight increase of transported mass and volume. A self driving trailer with washing and sleeping facilities could enable digital nomads on the road. A further improvement would be transmitting energy from stationary power stations with small motors in the vehicle and correspondingly less mass and lighter construction, offsetting the increased comfort and office weight. Such stationary energy could be more environmentally friendly than combustion and build up in a slow transformation process from fossil fuels to storages of renewables.



as i often drive in France vs Germany in exactly same car i can tell it indeed makes difference at the pump at wchich speed You travel.

France has more roads with 80km/h limit, in Germany highway pace is usually 160km/h.

I use LPG powered car with short transmission so travelling over 150km/h is really not economical, but i used similiar car with modified transmission and got better, but still similiar results.
So for French roads i get about 6.8L/100km at 80km/h, in 900kg car.
In Germany, same car running at 140-160km/h consumes 9L/100km.

Going below 6.5L/100km is not really possible by slowing down to 60km/h though. Even with best eco-driving and slowest cruises best i could get was 5.77L.

I can admit though then WHEN you are not in hurry this saves loads of money.
OTOH i did cross Germany in one go many timrs and i admit that option to shave over two hours , up to four, is really making a difference. It makes it possible to reach destination in one day, and do something, while in France i need two days and and a nap for same distance.

By travelling around Austria and Czech republic i did learn other lessons though. Not all cars CAN travel over 60km/h in mountain terrain, and so one can get nice example how roads can get unsafe and congested at such speeds.
But then engineering to the rescue!
Roads in those countries have many amenities for slow cars, wchich are used not only by trucks, but by fuel(and mainteance...) aware passenger drivers too.

Those amenities are dead simple : extra slow lanes for any uphill possible, so fast cars are not blocked.
More lanes and decent in-advance markings.
Shortcut roads for slow cars (or roads parallel to the highway)

Such simple things really help, and even in difficult terrain i could keep my consumption in 7L/100km limit , driving safe and slow while at same time not disrupting traffic.

Worst things for fuel and car were German irbital highways around cities like Berlin, where one has to run 140km/h on SLOWEST lane. No way to let the poor engine cool by sime coasting, and if anything like someone butting in right in front of you by changing his lanes slowed You down, one had to step the gas pedal to the floor to be not honked by trucks behind. 9L/100km...

Unfortunatelly slow lanes cost concrete and land. They are needed most in places where land is at most premium - where most people drive daily, around cities.

OTOH i really like that in Germany there is whole network of cycle paths connecting towns and cities, Independent from road system. It really helps if You can go Somewhere by bike. They reach city outskirts, popular destinations like supermarkets... France has almost none of those and cycling and motoring under 60km/h is not so much possible.




I keep coming back to this article and thinking about it in new ways. Today it occurred to me that the systemic effects of speed might amplify the benefits of slowing down even more.

A large fraction of travel is routine daily travel: trips to work, school, and the store. For that kind of travel, people tend to plan based on time rather than distance per se (at least, I certainly do).

While day-to-day travel decisions are made based on where things currently happen to be located, on the scale of years or decades the reverse is true. Businesses choose locations, people choose housing, and public infrastructure is located according to accessibility. While Beijing and New York will always be the same distance apart, your grocery store or office will probably be roughly the same time away in a century as they are now.

So for daily travel needs, we might as well consider energy used per unit time as per distance.

flow in


double speed = half distance is simply not true.

I often drive 200-300km for work.

I've experimented, between going at 120k and going at 80.
at the most, 10% more fuel use at 50% more speed.

engines have resonances, sweet spots. when the engine is working in its design point, when every part of the drive chain is moving at its most efficient speed, the fuel economy increases.

we don't drive bricks, with simple air resistance / speed curves.

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