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On UK roads, I frequently see cars driving very closely behind larger vehicles, attempting to stay in their slipstream, reduce drag and thus increase fuel efficiency.

Given the dangers of driving so close, are there any studies which demonstrate that the fuel efficiency savings are real and significant? If so, do they also study the road safety aspects of slipstreaming?

Oddthinking
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Mark Booth
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    Mythbusters did try this out -- I think it was confirmed. – Sklivvz Sep 27 '11 at 10:27
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    @sklivvz yep they did http://mythbustersresults.com/episode80 – ratchet freak Sep 27 '11 at 10:48
  • if you make it an answer, try to include some info until which distance there is a notable effect. – johanvdw Sep 27 '11 at 11:43
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    It's not just lorries (trucks). NASCAR drivers know that every additional car added to the tail end of the train helps improve the mileage of every other car in that train. – oosterwal Sep 27 '11 at 12:10
  • It's not just cars, [marathon runners](http://www.runnersworld.com/community/forums/training/marathon-race-training/deal-drafting) do it too. – Wayne Werner May 19 '12 at 14:39
  • Anyone who has every done this on a bike (called "drafting", I think) knows (from the feedback they get from their leg muscles) that it works. But the biker in front of you has to be working in coordination with you, or you're in for a world of pain when they brake surprisingly. Given that the laws of aerodynamics don't suddenly change when you're in a car, it's hard to conceive of it not working. The bigger question is: is it a stupid thing to do? I think the answer to that is probably "yes". – iconoclast Sep 14 '12 at 18:48
  • You don't have to drive dangerously close to a lorry to catch its slipstream, it will still be there a few car-lengths back. Anybody driving dangerously close is just a bad driver. – GordonM Aug 24 '15 at 08:28
  • If you have any source which provides evidence for this @GordonM please add an answer. I think that it is likely that if you are two seconds behind the 'big rig' then you are as likely to be caught in turbulence behind the rig as you are to get any benefit from any kind of slip-stream. – Mark Booth Aug 24 '15 at 12:41
  • Being from the UK and having known people that habitually drive a couple of feet behind other vehicles, including lorries, even at high speed I'd say that this had more to do with encouraging the driver in front to get out of the way, or to prevent someone else pulling into the gap between the vehicles, rather than an attempt to save fuel. – daveb Apr 11 '17 at 08:57
  • Given that lorries are usually going 10mph slower than the rest of the traffic @daveb I wouldn't expect your suggestion to be common, but if you have any evidence to back up the claim, feel free to post an answer. – Mark Booth Apr 11 '17 at 13:54

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mythbusters found out it can increase fuel efficiency with 40% at 10 feet behind the truck http://mythbustersresults.com/episode80

Drafting behind a big rig will improve your car’s fuel efficiency.

confirmed

To test this myth, the build team procured a car, a big rig, and a device that could measure a car’s fuel efficiency. They then drove the car behind a moving big rig at various distances ranging from 100 to 2 feet and measured the amount of fuel the car consumed. The Build Team discovered that the closer the car was to the big rig, the less drag is produced, thus the more fuel saved. At just ten feet, the car managed to increase its fuel efficiency by 40%. Drafting at two feet was slightly lower than the ten foot distance, mainly because Grant had to keep working the car pedal to maintain distance from the truck. However, that did not dispute the fact that drafting actually can increase your car’s fuel efficiency. However, the Build Team has warned that drafting is incredibly dangerous because the truck driver may not able to see you and you may not be able to react in time if the truck were to make a sudden stop.

the only video I was able to find was a 240p here you can see the full table http://www.youtube.com/watch?v=lttgT1XZVvE&feature=player_detailpage#t=534s

you can see that the economy starts at 100 feet with 11% saving over the control than as you get closer the economy increases up to 40% at 10 feet

ratchet freak
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    Note that Mythbusters are not considered authoritative here: http://meta.skeptics.stackexchange.com/questions/450/is-mythbusters-ancedotal-evidence – Sklivvz Sep 27 '11 at 12:55
  • Thanks, it's interesting that an 11% saving requires [drafting](http://en.wikipedia.org/wiki/Drafting_(aerodynamics)) at 100ft (30m), so at 70mph (112kph), that is just 1 second behind the vehicle in front - half the [safe distance](http://en.wikipedia.org/wiki/Two-second_rule). To be safe, you'd have to be doing 35mph, but at that speed I doubt that would create as powerful a slipstream at 30m, so the savings would most likely be even less. – Mark Booth Sep 27 '11 at 13:03
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    @Sklivvz - Agreed, but it is a more valid data point than anecdotal evidence or pure speculation. It would be nice if anyone knew of any more rigorous studies, certainly the references of the wikipedia [drafting](http://en.wikipedia.org/wiki/Drafting_(aerodynamics)) page are no better or more relevant than the mythbusters reference. – Mark Booth Sep 27 '11 at 13:07
  • @mark unfortunately that is the most rigorous test I've encountered on the web through a quick search and is cited by most blog posts about it – ratchet freak Sep 27 '11 at 16:33
  • Remember this works even for slower vehicles, such as bicycles - you just have to look at the peloton in the Tour De France. There everyone quotes figures of 25-30% energy savings on the flat when 6 inches from the bike in front. – Rory Alsop Sep 27 '11 at 21:12
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    @Sklivvz I recall that in this episode, they had aerodynamicists perform scale-model tests in a wind turbine, the Mythbusters' experimental results confirmed these findings. Still not a study, and further references would be good, but this appeared to be one of their more procedurally thorough experiments. – John Lyon Sep 27 '11 at 23:04
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    @Rory A bicycle can equally draft behind a truck quite well. http://www.youtube.com/watch?v=Ss3fjLaCrRM – John Lyon Sep 27 '11 at 23:06
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    yes, this was a well planned and executed experiment by them. I doubt any scientific institution is going to test this, as there's no scientific interest in doing so. Road safety aspects mean that no agency like the AA is going to test it either (as the distance you'd need to maintain is definitely, as stated multiple times by the Mythbusters) NOT safe). So this is probably the most rigorous study out there, even if not executed under totally controlled conditions. – jwenting Sep 28 '11 at 07:08
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    @Rory, the point that I was making was that since the benefits drop off the further you are from the vehicle in front, would you get any measurable benefit if you were a *safe* distance behind them? I'm sure we've all seen those catastrophic domino effect pile-ups during the tour-de-france, where everyone in a block of *6" behind* racers can't react quick enough to avoid the person on front of them coming off. – Mark Booth Sep 30 '11 at 10:39
  • A younger me used to do this, very dangerous. I suspect that there is also a suboptimal distance where the 'wake' of the truck's passing is more turbulent. – Benjol Aug 24 '15 at 10:02
  • Guy Martin slipstreaming a bicycle behind a truck at 110mph: https://www.youtube.com/watch?v=wT0eJXtrwHg – Roger Lipscombe Apr 11 '16 at 10:54