Both “click” and “klick” are slang for a kilometer. But, which one is in more common use worldwide?
In American military usage, “klick” appears to be the more common spelling. However, in at least one book I’ve read by Arthur C. Clarke (a British author who lived in Sri Lanka), he spells it “click”.
Is “click” the more common British spelling?
In the war of click-spelled kilometers vs. klick-spelled kilometers, who wins?
If this were a poll I’d vote “klick.” Since it’s not a poll, all you have is my opinion/guess.
Klick. Kilometer begins with a “K” after all.
Speaking as someone who works with large linguistic corpora every day, you’re never going to get an accurate answer to your question, unless you have a lot of money to dish out for research grants. Because the word “click” (and maybe “klick” too) has meanings other than “kilometre”, you can’t just use a web search engine to compare hits. (Well, even if the words weren’t polysemous, you still wouldn’t be able to do that.) You’d need to get a very large, representative sample of English documents and run a word sense disambiguation program on them to weed out all the ones where “click” and “klick” aren’t used to mean “kilometre”, and then count the remaining ones. Such a corpus does not exist—at least not if you want your results to be statistically significant. (There are lots of very large corpora, and probably lots of corpora which reflect the real-world proportion of “click” to “klick”, but probably none that fulfill both criteria.)
Can’t you at least ballpark it for estimation, by searching on, say “click means kilometer” and “klick means kilometer” and see what you get? I get 3 Google entries for the first, 56 for the latter? Then “click means kilometre” and “klick means” kilometre" returns 4 for the first, and 3 for the latter. Then “click stands for kilometer” and “klick stands for kilometre” gives me 1 and 3, respectively. I don’t necessarily mean those choices specifically, but something of that nature, where you’re only going to catch a small percentage of “clicks” and “klicks” that mean kilometer, but (hopefully, and this is the big assumption it hangs on) a representative sample of the greater usage of the words?
Nitpick and story: The British would probably spell it “kilometre.”
In my circles we don’t really use c/klick as slang for kilometre, but pronounce it quickly as one long word /claw-mitter/. I tend to find the pronunciation /key-low-meter/ a bit annoying, but less annoying then the slang “c/klick” (which I can’t stand).
I suppose you could shorten it to “kilo”, but then that would be easily confused with a certain unit of mass. 
How about if you pronounce it “kay-em”?
There’s no established slang term for it in British English, because we don’t really use kilometres.
I call it a metric mile.
Then how do you talk about episodes of Star Trek: The Next Generation?
I mean, the seem to use metric units excusively on that show.
(Oh, wait, I’ll bet I know – even with ST:TNG, kilometers rarely come up, because starship battles seem to be fought at distances of a few feet. I swear, those guys have no sense of how far apart things can be in space.)
The usual slang term in Australian English is “kay”, or K. I don’t think it has an established spelling because it’s not commonly written.
This has little to do with the issue, but I was a long time associating c/klick with kilometer. For the longest I thought it had to do with an adjustment on a mortar or field gun, maybe even a sniper rifle. Once I learned it had to do with a kilometer I was let down and unimpressed.
According to http://usmilitary.about.com/od/theorderlyroom/f/faqklickdef.htm, “click” (spelled with an initial c) does have to do with an adjustment on a sighting system. It’s “klick” (spelled with an initial k) that the military uses when discussing kilometers – and even there, there’s a chance that the term had its origins in the clicking of an odometer or some such.
EDIT: Of course, when you hear some guy playing a soldier in a movie say “The target is 5 {c|k}licks downrange, sarge!”, he’s talking about the kilometer version, not the sighting-adjustment version.
If you take the entirety of Google’s index to be a representative sample of English, sure. But the problem is that it’s not, and when you’re dealing with differences in frequencies as low as 1 or 2 hits among several billion, comparing them isn’t going to yield statistically meaningful results anyway.
American drug sellers and users used to use “key” to mean kilogram, because that’s how the pot was weighed. However, my info might very well be out of date.
As for the original question, I’ve always thought of it as a klick, to distinguish the length from a short sharp sound.
You shouldn’t. Scandinavia already has metric miles and they’re 10k. And, in case the OP really wanted a world wide survey, to us klikk never means a kilometer.
Would military literature really use ‘klick’ instead of km or kilometer? I’d have thought it was just verbal slang often spoken but seldom written like brain bucket for helmet or megs for megahertz.
To my eyes, “klikk” (spelled that way) sounds like the name of an invading alien insect species from a video game.
In German, neither - no appreciable slang term use; we just use Kilometer in casual as well as formal contexts.