The only abbreviations I've ever seen in technical articles / journals are KM for Kilometers, and KG for Kilograms. In the military the slang for kilometers is klicks.
"Showing" as in dialogue presents an issue... "miles" is easy to say, but "kilometers" can be a mouthful (well for me at least because I live in the US). So if the speaker lives in a metric world, do they say "Boulder is 165 KM from here", or do they say "Boulder is 165 Kilometers from here"?
Considering that my readership will likely be mostly in the US, I need to be careful with temperature as well. He was sweating like crazy. Damn, it must be 50 degrees out here! You see, I have to let readers know that I'm referring to celsius (50 C = 123 F). In the example I gave, it was thought, so I could add the 'C', but in dialogue it would be awkward... I for one don't say, "Dang it's hot... the thermometer says it's 123 F out here." So I can't have a character say, "Dang it's hot... the thermometer says it's 50 C out here."
I say "degrees", or even just the number if it's negative, e.g. "it's minus five out there". This is probably the same for people who think in Fahrenheit but you can usually tell from the context if someone means Fahrenheit not centigrade/Celsius. e.g "she's got a fever of 39.6. I'm calling NHS 111" (Non-emergency NHS number.) Over here, everyone uses centigrade so there's no point saying it's centigrade, same as people in the USA don't bother saying it's Fahrenheit, they just leave it for the rest of the world to figure out seeing as someone would be dead long before their temperature's 98 degrees and storing food at 35-40 degrees is a very very very bad idea.
If you make it clear early on that it's centigrade not Fahrenheit, trust your readers to figure it out. 50 Fahrenheit isn't hot, so when your character says "it must be 50 degrees out here" and you've made it clear that they feel like they're being burned alive when they step into direct sunlight, or that the wind feels like a hair drier set to hot, or that if you go out with bare feet the soles of your feet get burned on your patio, it's blatantly obvious the character's talking in centigrade not Fahrenheit.
BTW I used to live in Saudi and it got that hot every day during the summer.
So - the core of my issue is that I'm writing a story of a world where the metric system is standard, for readers where the english system is the norm. Sometimes I want to toss it all aside and go with furlongs and stones. (just kidding).
What I'll probably go with for temperature is to add the "C", for distance use "KM", and for speed use KPH in both thought and conversation. It still feels a little awkward, but maybe I'll get used to it.
km shouldn't be capitalised. The speed should be written km/h. K (capital k) is Kelvin, which is a unit of temperature. Small k is for the kilo prefix, meaning 1000x something.
Note: I just checked online, apparently some publishing houses prefer kph however the SI unit is km/h. Personally, I've never seen kph and it looks weird and I wouldn't have known what it meant if it was outside this conversation, because kilometers is km, not k. I've seen kmph used but not kph. The SI unit system is an internationally agreed system that's used by scientists all around the world.
As I said above, I wouldn't bother adding the C for centigrade as no-one actually says it unless they're talking to Americans. Even then it's more like "yeah when I was in Saudi the temperature was over fifty every day for the whole summer. That's centigrade by the way, not Fahrenheit." And if it's obvious it's temperature you're talking about, there's not a lot of point even saying "degrees".
- - - Updated - - -
I am curious because I have never heard "kays" or "kay-gees" spoken colloquially. (I lived for almost a decade in Canada.)
Colloquially in the UK, it's kilos.