# In Back to the Future, why was the speed 88 miles per hour?

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

Unit Equivalent Abbreviation
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 about this number. So why was it 88 MPH?

• 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. Apr 13, 2012 at 20:24
• Must be for the yards per second metric - I see 42 in there.
– Iszi
Aug 17, 2012 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. Sep 27, 2012 at 16:54
• @JanusBahsJacquet 70 MPH is more common on highways in America because there was originally a national speed limit of 65 MPH on all highways. It's been repealed since 1995, but some states (including mine) still have a 65 mph speed limit. Other states have higher or lower limits, but most aren't any higher than 80, so 88 would be considerably fast (and faster than legal in 1985, when a 55 MPH limit was the national law en.wikipedia.org/wiki/National_Maximum_Speed_Law ) Jul 3, 2015 at 1:11
• The point is that it's 236,544 furlongs per fortnight. Dec 5, 2016 at 9:57

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

Further evidence on Wikipedia supports this.

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

• so many cars of the 1980s had speedometers that only went to 85, that the case for "up to 11" is strong. May 2, 2012 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). Jun 17, 2012 at 23:05
• @JustJeff : :) I can't resist... about the "up to 11": xkcd.org/670 Aug 28, 2012 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.
– user8693
Feb 27, 2014 at 23:09
• Luckily, I could smell the TVtropes link from a mile away so I managed to avoid clicking on it. The day is saved! :) May 1, 2016 at 9:36

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.

• +1 for "has a certain alliterative/poetic rhythm to it." Eighty-eight miles per hour has a certain ring to it.
– user20155
Feb 21, 2014 at 3:42
• FWIW, 88mph is one of the most common speeding tickets (historically) in the US because there's a "lamp test" switch on many 7-segment radar units that lights up the entire panel of LEDs, including all of the seven segments of each digit, so that the officer can point out to the victim that the radar really showed them going 88mph. This is probably not as true now as it was in the 1980s. Jun 21, 2023 at 13:51

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.

And there's also an interview here where he adds that aside from being memorable, another reason was that it was fast enough that the characters wouldn't accidentally go that fast when just driving the DeLorean around (and not trying to escape Libyan terrorists):

Two reasons. First of all we wanted it to be a speed that somebody wouldn't accidentally drive at. The other thing is, is it's easy to remember. Everybody remembers 88 miles an hour. Maybe you'd remember 89 miles an hour. I don't know, 88 just had the right ring to it.

• A word of god answer. Can't beat that. Nov 13, 2014 at 21:28
• I'm still so new to actively participating on SE that stuff like this takes me aback. I mean, democracy is all fine and dandy, but what the hell? Here's another case of a resourced, researched, documented answer not the accepted answer. I know people don't want Wikipedia style edit wars (much appreciated, that!), but seriously, is there remedy here beyond flagging stuff? May 1, 2016 at 11:51
• @Marakai - Keep in mind the question was posted in April 2012 and an answer was accepted one day later, while I didn't come across the question and post my answer until over 2 years later in 2014...my answer has been gaining in points since then, but Rodrigo, who asked the question, doesn't seem to have been active on this site for a while, so he may not have seen my answer. This is sort of a drawback of the "only the original question asker can choose what answer is accepted" system, but see this question for some ideas about workarounds. May 1, 2016 at 20:18
• @Hypnosifl thanks for that. I like the approach with a new question and flagging the old one. This is also quite topical as I had started a question on History Meta: there it was a bit more "serious": answers where at least parts were sourced from far-right propaganda websites were being accepted. In those cases flagging is another option, but it requires some time and patience. May 1, 2016 at 20:50
– wra
Jun 21, 2023 at 20:28

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.

Edit1: 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.

Edit2: People ask: Why is it that we account for the rotation of the earth but not the movement of the earth around the sun? Why doesn't the car appear in outer space? Simply put, the law of conservation of energy keep the time-travelling car earth-bound. If you lift an object, it gains potential energy. This energy has to come from somewhere. If you move a car from earth's surface to outer space, it gains A LOT of potential energy.

How much? The potential difference between the earth's surface and faraway space is given as GMm/r where G is the gravitational constant, M is the mass of the earth, r its diameter and m is the mass of the car. The nominal weight of a Delorean DMC-12 is 1230 kg, which equates to an outer-space potential of about 77 Gigajoules. More or less the energy expended by a car in a whole year. There is nowhere for this energy to come from.

The law of conservation of energy therefore stops the car from translating to outer space. Put another way, the car follows the most energetically favourable path along the 4D spacetime gravity well. Similar to how water flows along bends in a pipe, without spending energy on turning around corners.

• 0_0 (that is all I wanted to say) Feb 28, 2014 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, 2014 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. Feb 28, 2014 at 21:20
• +1 for what is either the most obscure intentional explanation of a number ever, or the most amazingly awesome coincidence ever. Jul 2, 2015 at 21:26
• This answer is awesome, except that Earth not only rotates but also moves through space (at about 107,000km/h). But still, awesome explanation. Oct 20, 2015 at 0:07

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:

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.