You would use kilometers.
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Kilometre.
I would assume that you would use cm or even mm if you wanted to be more accurate
Ex. the distance between two cities A kilometer is 1,000 meters
If the cities are close to each other, then you can treat them as if they were on a plane surface and the distance between them would be the scaled-up version of the distance between them on a map. However, if the cities are further apart then you need to take account of the fact that they are on a spherical surface. To do this you need to identify the plane which is defined by the two cities and the centre of the earth. This plane will meet the surface of the earth in what is known as the great arc, and the distance between the two cities is measure along the great arc. Incidentally, this is why flights from London to Tokyo, for example, fly over the North pole.
The customary unit would be miles (mi), and the metric unit would be kilometers (km).