A US customary mile is 5280 US customary feet. 1 US customary foot is 12 US customary inches. 1 US customary inch is 25.4mm. So a US customary mile is 1609344mm, exactly. It derives from the roman "mille passus", literally 1000 paces, where a pace is the distance between two impacts of the left (or right) foot of a Roman soldier on the march. Quite a few other cultures used a "mile" of some sort even after the fall of Rome, for example the old British imperial mile was 1760 British imperial yards, one British imperial yard predated the definition of the meter but was most precisely measured to be 0.914398415m, so the British imperial mile was 1609341.21mm. Other culture's miles varied even more than this.