It could be using your phone/browser language settings; try English (Australia) rather than English (United States).
Ah, thank you, this was it. I had the language set to English (UK), but changed the distance setting to KM. I got the app wrong, it was Uber, and Uber doesn't respect the override, so it always uses miles. Changing it to English (Australia) and Uber switches KM.
In Gmaps, Tap your profile picture, then select "Settings" and "Distance units". Choose between "Automatic", "Kilometers", or "Miles".
Pick the units you want.