For me, it's normal to write "its mileage is 165k" - and the "km" is assumed. I just wanted our friends that use imperial units to understand, you're true it's non-standard, and it's actually the first time I wrote it like that.
Well Americans did (sometimes still do) use micro-micro, as in µµF.
I guess it's less confusing than using "mm" for "megameters". "I drove my car 266mm" doesn't sound like much of an accomplishment.
mm is millimeters though, so that would be about a quarter of a meter. I guess Mm would be what you are looking for.
The capital letter doesn't add much to the sound of it.