That must be why I could not find the post I was looking for. I recently upgraded my car head unit from one with just the radio and the CD player to one with about a million functions including a kitchen sink. Amongst other things, it also does navigation. Which is what has brought me here. I recalled that there was a discussion here somewhere about what units to use apart from metric. This new unit of mine used miles/feet by default and it just felt so alien that I was going to switch to metric, despite all the road signs being in miles. Luckily there were three options: metric, miles/feet and miles/yards, just like you eventually concluded