I was once riding with a cyclist who had not been riding for long and had just acquired a speedometer. I've never had a speedometer on my bike. I asked him how fast we were going and he said "35." I knew that was way way way too fast at the moment (flat land and not max effort). I told him that his speedometer must be reading in kilometers. Kind of ruined his day. He thought he was pretty fast before I ruined it for him.
Some people like to have their speedometers in kilometers because the race distances are measured in kilometers. Just watch out before you tell your friends that you went on a ride this weekend and averaged 35mph when you mean 35km/h...it could be an interesting material to make fun of you.