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This might seem familiar to many of the veterans here, but since it has not been exclusively asked, I am posting it.

A 13-foot tall truck is at the entrance of a tunnel with a clearance of 12 feet 11 inches.

What is the most economic and simplest solution possible that the driver of the truck has at hand to pass through the tunnel safely to arrive the other side with the truck?

N.B. As the problem implies there are multiple solutions possible. I am expecting a specific answer which on consensus of the reader(s) can be regarded as the acceptable answer.

Disclaimer: I did not create this puzzle but heard it way back in the 80's as a young kid. I think it is a Edward De Bono puzzle.

Anyways the alternate answers are rarely documented which is what I am after. Investing further time to curate through the list of answers is no longer possible and am accepting the classical answer.

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closed as too broad by Aza Aug 3 '15 at 17:51

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ Having watched far too much Looney Tunes as a child, I suggest the following: 1) paint the wall above the tunnel to look like the interior of the tunnel, giving an (illusory) extra foot or so. 2) find a coyote and induce it to chase you. 3) Drive through the now 14' tall tunnel. 4) (optional) laugh as the coyote runs into a painted wall 5) meep meep $\endgroup$ – LogicianWithAHat Aug 3 '15 at 15:50
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    $\begingroup$ My first instinct is to say "it cannot possibly do so" because my default picture of a tunnel is circular (or at least, say, the upper four-fifths of a circle)... if it's 13' high at the center, a 13' truck would never get through. $\endgroup$ – TripeHound Aug 3 '15 at 15:56
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    $\begingroup$ A version of this puzzle - complete with Bailey M's answer - was included in the film "Working Girl" (1988). Might this have been where you originally heard it? $\endgroup$ – user9278 Aug 3 '15 at 16:04
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    $\begingroup$ Would it be possible for you to edit this question to limit the scope of possible solutions? This could potentially be done by more specifically defining what you mean by economical/simplest, or by limiting the possible scope of out-of-the-box answers. $\endgroup$ – Aza Aug 3 '15 at 17:54
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    $\begingroup$ Get a great number of migrants to get inside the truck and weigh it down. This would work very well at the moment with the Channel Tunnel. dailymail.co.uk/news/article-3183668/… $\endgroup$ – chasly from UK Aug 3 '15 at 19:58
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My first instinct would be to

Let some air out of the tires, so that you end up riding an inch or so lower and can pass through safely, albeit maybe a bit bumpy.

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  • $\begingroup$ Excellent !!!!!. $\endgroup$ – kanchirk Aug 3 '15 at 15:22
  • $\begingroup$ This is, in fact, the classical answer to this riddle. $\endgroup$ – Dan Henderson Aug 3 '15 at 16:34
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Assuming the measurements are completely accurate, then a solution is:

To drive the truck through the tunnel! For peace of mind you might want to grease the roof. As long as the tunnel is uniform, and straight, and the truck doesn't move in such a way as to ever increase its height, then a 13' object should fit through a 13' gap.

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    $\begingroup$ +1 This was actually my first thought. I had to read through the comments before I figured out what the problem was supposed to be. $\endgroup$ – Sabre Aug 3 '15 at 17:57
  • $\begingroup$ I've edited the problem to make it more in line with the versions of the problem I've heard, which make it less ambiguous by using terms like "clearance". $\endgroup$ – Joe Z. Aug 4 '15 at 5:51
  • $\begingroup$ @JoeZ. Fair enough, I think that must have been a difference between this and other versions of the question :) $\endgroup$ – Samthere Aug 4 '15 at 7:55
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Many large trucks have:

Adjustable air bag suspensions which can be lowered. Simply lower the ride height and drive right through.

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Blast the A.C. so the truck shrinks a few mm.

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  • $\begingroup$ Please add an explanation of how blasting the air conditioner would shrink the truck, or this answer might be deleted. $\endgroup$ – JLee Aug 3 '15 at 16:25
  • $\begingroup$ @JLee things get smaller when they're colder. en.wikipedia.org/wiki/Thermal_expansion $\endgroup$ – Nick T Aug 3 '15 at 16:34
  • $\begingroup$ an air conditioner removes heat, so, part of it is giving off heat equal to the amount of cold it is giving off, so there is no net change. but even more importantly, would changing the temperature inside a metal framed truck by 10 or 20 or even 30 degrees really be enough to affect the size of the truck? $\endgroup$ – JLee Aug 3 '15 at 16:39
  • $\begingroup$ suppose the temperature is 18 C, or 64 F. Then it would shrink by about: ∆L = α∆TL = 12e-6 x 18 x 390 cm = 0.0846 mm $\endgroup$ – nietsnegttiw Aug 3 '15 at 16:51
  • $\begingroup$ @nietsnegttiw It would be expensive. $\endgroup$ – kanchirk Aug 3 '15 at 19:44
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Since we know that tunnels are mostly shaped like a circular tube, a 13-foot truck cannot pass from a tunnel that is 13-foot or lower. This can be further calculated, by estimating width of the truck, regular tunnel shape and so on.

An economical approach can be changing suspension height, just like people who do lower their cars. If that is not enough, rims can be changed and tire pressure can be decreased.

Although 13 foot is not that high. Most small trucks are lower at size. Best thing to do would be putting the load sideways with some help from cranes. It would not be perfectly stable, but it will do the job.

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