I live in the USA. We use miles. If the rest of the world doesn't like it, keep in mind that WE'VE actually used a nuclear weapon before.

Since I've spent my whole life talking in MPH, that's what I use. When I watch a bike race, I just multiply the speed times .6 in my head to get a good estimate. Here's a snapshot: When I hear "The peleton is averaging right around 45 km/h", I can quickly figure out that 6x4=24 and 6x5=30. So 45kph is halfway between 24 and 30, or about 27mph. You'll also notice that I drop the decimal point from .6 and just deal in 10's of kphs....i.e. .6x40=6x4. The actual answer, by the way, is 27.9mph, so I'm off slightly, but I'm ok with an error factor of +/- 1mph since it's safe to assume that the announcers are probably similarly inaccurate.