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Why not use 89, maybe 100 miles per hour? Is 88 the fast a DeLorean can go? In other metrics, I have:

Miles per hour         88.00    [mph][mi/h]
Miles per second        0.0244  [mps][mi/s]
Yards per second       42.944   [yps][yd/s]
Feet per second       128.832   [fps][ft/s]
Kilometers per hour   141.3648  [km/h][kmh][kph][kmph]
Kilometers per second   0.0393  [km/s]
Meters per second      39.3     [m/s]
Knot                   76.3931  [kn] nautical mile per hour
Mach Speed              0.1155  [Ma] (speed of sound)
Light Speed             1.3110e-7 [c]

There does not appear to be anything special with this number. So why was it 88 MPH?

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I'm guessing because the led display would seem more dramatic if all the lights were lit up and since 88 is the number that would do this, it was what was chosen. –  OghmaOsiris Apr 13 '12 at 20:24
Must be for the yards per second metric - I see 42 in there. –  Iszi Aug 17 '12 at 6:06
@OghmaOsiris - I think you can even take this explanation in-universe. Imagine that the speed is arbitrary - you fix it by changing variables in the machine...Doc has to come up with an arbitrary large speed, and since he's using an LED display for the spedometer, he decides it would look cool. We know that Doc has a flair for the dramatic anyhow. –  Chris B. Behrens Sep 27 '12 at 16:54
@ChrisB.Behrens: precisely. Because it’s cool. –  Paul D. Waite Nov 30 '12 at 14:20
5280 ft in a mile. 5280/60=88 –  user1873073 Jul 25 '14 at 2:28

6 Answers 6

up vote 87 down vote accepted

As you can see here the original set of Deloreans were fitted with speedometers that went up to 85 MpH:

A picture of a Delorean dashboard; the dial speedometer goes up to 85mph / 140kph.

Further evidence on wikipedia supports this.

The fact it needs to reach 88 MpH may be an indication that the Delorean is supped up, or that time travel is impossible. Or even just it needs to be turned up to 11 to get time travel.

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so many cars of the 1980s had speedometers that only went to 85, that the case for "up to 11" is strong. –  JustJeff May 2 '12 at 0:05
That's because in 1979, the NHTSA made a regulation limiting speedometers to 85 MPH (as well as special emphasis on 55, which was the national speed limit at the time). –  dan04 Jun 17 '12 at 23:05
@JustJeff : :) I can't resist... about the "up to 11": xkcd.org/670 –  woliveirajr Aug 28 '12 at 12:02
@Pureferret The 140 mph speedometer was for cars intended to be sold in the UK and Europe; at the time US regulations required that speedometers top out at 85 mph. DMC now has a replacement 140 mph speedometer and metric cluster for sale. –  Michael Hampton Feb 27 '14 at 23:09
This would be a good in-universe explanation for why Doc's Delorean had an extra digital speedometer mounted on top of the dashboard (out-of-universe, they probably just put it there so the audience could easily see the speed in point-of-view shots looking out the windshield) –  Hypnosifl Nov 14 '14 at 15:15

I think that the answer is in four parts:

  1. It had to be fast enough to be dramatic without being extreme.

  2. OghmaOsiris may be right - 88 would completely fill an old style LED display.

  3. Eighty-eight has a certain alliterative/poetic rhythm to it.

  4. The number 8 is the symbol for infinity rotated 90 degrees.

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+1 for "has a certain alliterative/poetic rhythm to it." Eighty-eight miles per hour has a certain ring to it. –  Lego Stormtroopr Feb 21 '14 at 3:42

An in-universe explanation with a bit of math makes it all clear:

A Delorean DMC-12 is 4216 mm long. When travelling at 88 mph, the car then travels its own length in 4216mm/88mph = 107.2 ms. So this is how long the time-travelling wormhole-thingy that opens in front of the car has to be open, or alternatively the minimum time the flux capacitor is actually in effect. Could this time interval be significant?

Note how the Delorean arrives in the same location on earth after travelling in time, but can arrive at different times of day. Assuming it is gravity-bound it must still somehow be able to translate along the circumference of the earth, to correct for Earth's rotation. We know that the time-travel takes place in California, which is at 37 degrees north latitude. If you travel due east from 37 degrees north and circle the earth, the distance travelled is "circumference of earth" * cos(37) = 32 005 km. Now notice that light travels this distance in 32 005 km/"speed of light" = 107 milliseconds!

It is then clear that Doc Brown uses the speed of the car to modulate the duration of travel, but in space, not in time. With reference to Minkowski space-time, the car leaves its normal time-like curve for a spacelike but performs a translation in space when passing through its lightcone, where it attains exactly the speed of light.

107 milliseconds affords travel to any point in time while returning to the same point on earth. A round-trip might be necessary depending on whether you travel forward or backward in time. If the car had been at the equator, the car would have to travel at only 70.38 mph. This would actually be a disadvantage as Doc Brown would have to provide more energy to keep the wormhole open for longer.

Not that the energy requirements are that large actually. The Delorean is stated to require 1.21 Gigawatts for time travel. Watt is Joule per second and 1.21 GJ/s * 107 ms = 130 megajoules. This is about the energy released by combusting one gallon of gasoline. A gallon per trip makes for good mileage on a time machine.

Edit: As Stephen Collings points out in a comment below, this need to expend energy very quickly also explains why the Flux Capacitor, according to the professor, is so central to the time machine's design. It's not the energy requirements themselves that's the hard issue, but the ability to release it all in an instant, and within the confines of a car no less.

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0_0 (that is all I wanted to say) –  Navin Feb 28 '14 at 2:18
Assuming the writers actually did this math to come to the magical number and didn't just say "uh, sure, 88 mph, whatever" hella-props to them! –  Dave Feb 28 '14 at 16:26
@Dave: Sometimes I'm thinking guys like us in the scifi forums have put more thought into the movies than the scriptwriters did. –  Abulafia Feb 28 '14 at 21:20
I barely understand a word, however it sounds pretty solid to me. –  Bardo Nov 14 '14 at 13:46
130 megajoules isn't much energy, but dissipating all of it in a tenth of a second still pretty hard. Capacitors might have that kind of power output, but then the volume needs go up for the energy storage. You'd need ultracaps outmassing the delorean. That flux capacitor really does make time travel possible! –  Stephen Collings Jan 16 at 3:11

Some nice speculations here, but according to producer and co-writer Bob Gale, they only picked it because it was a memorable number. On the Back to the Future blu-ray (probably the DVD too), if you go to the "Extras" menu and turn on "Q&A commentary with director Robert Zemeckis and producer Bob Gale", then play the movie with the audio from the Q&A, if you skip ahead to 26:38 in the movie you'll hear Bob Gale say:

The fact that everybody says, "Why 88 miles an hour? What's so special about that?" It's easy to remember. That's all. There's no special significance to that.

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A word of god answer. Can't beat that. –  Richard Nov 13 '14 at 21:28

Not covered in the other answers, I believe viewer safety has to be factored in.

A driver of average competency begins to seriously increase the risk of crash when 20kph over the speed limit, as shown in this figure:

enter image description here

If they made it lower, it probably wouldn't have been cool enough to take effect. But if they made it higher, they'd run the risk of people actually trying to do it.

And honestly, they can't buy the sequel if they're dead.

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I had heard once that when they were designing the 5th-wheel coupling between 18 wheeler's tractor unit and the semi-trailer, it was observed that the design actually had an unstable transverse (sideways) mode of vibration which would become dominant around 88mph. At that speed, the semi-trailer starts swinging to left and right behind the tractor unit. This usually results in loss of control and crash. I think they got the number from there if not from the more appealing "infinity at 90 degrees!" one :D

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Any evidence other than you "heard this"? I've heard a lot of stuff. –  Mark Beadles Nov 29 '12 at 17:22

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