Title text:
They rounded down to 182.8 instead of rounding up to 182.9 because 182.9 might make the statement incorrect.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3248/
this post was submitted on 21 May 2026
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So, there's a Department of Homeland Security sign near the parking lot at the ferry.
Speed Limit 4 1/2 Miles Per Hour.
I always thought that was stupid.
Then I saw this comic and realized it must have been in KPH originally.
So I translated it myself.
7.242 KPH.
It's a boat though, so must be knots.
3.91
Keep going. We're onto something
2.012 m/s
.0000000067102425 * the speed of light
But then those miles in the GP would be nautical. So 4 1/2.
If you've 24" tyres and a first gear ratio of 3.5:1, then:
RPM x (pi x 24" circumference )x (1/63360 miles per inch) x (60 minutes per hour) x (1/3.5 ratio) = 4.5 mph
Giving the RPM at a satisfyingly round 221.6 rpm.
I don't know whether to be impressed or appalled.
You should send this comment to the creators. Seriously, I'm sure they'd get a kick out of it.
Also, your ratio rant reminds me of a story.
When they were making '2001 : A Space Odyssey' they had a problem. In the original story the mysterious object was a glowing diamond. The special effects team couldn't come up with a really good diamond, so they replaced the diamond in the story boards they'd already drawn. The just covered every diamond with a black rectangle.
Eventually, someone pointed out that the black rectangle looked really cool and otherworldly. So they replaced the diamond with the monolith we're all familiar with.
For years, fans speculated as to the deeper meaning of the monolith. Was it a tombstone, a book, the Bible?
Finally, the author was speaking and at the Q+A a young man stood up and declared he'd solved the puzzle.
The ratio of the monolith was 1 : 4 : 9, the squares of the first three numbers.
Arthur C. Clarke was so impressed he used the idea in one of the sequels.
I think they sometimes do that just to get people’s attention. You don’t normally see fractional speed limits so you pay more attention.
But you see 5 mph signs all over. It's in a parking lot, so it's still redundant.
Actually, they do this so helmspeople pay attention to the sign and the speed limit. Saying "5" makes people think they can squeeze up to 7-8, etc. That little extra speed might mean something in a busy boating or shipping lane.
That may be true.
However, the sign is in a parking lot as people drive up to an inspection gate, not on the water.
I edited my original comment to make that clearer.
I found it!
It was originally 2,011,680,000,000 picometers per second
Do speedometers even measure such low speeds with anywhere near that precision?
Depends on the car. I'm sure you've got high end cars with micrometer accuracy.
Realistically, if you're driving in a parking lot you should never be looking at your speedometer. Kids, dogs, shopping carts and other hazards jump out with no warning.