Source
As for my sources various places but chiefly a historical study of ancient measurements by Donald Kingsbury. He notes that the "mile" was originally one second of arc (~ 1854 metres), some 5,280 "feet" but not English feet - the conversion factor is 6080/5280 - yet that was a common error. Columbus used the reduced value because it made Indian and the East accessible by ocean voyage, whereas the real circumference meant they would die of thirst or starve, if it was all open ocean.
Forutnately it wasn't, and Columbus probably knew that - from Celtic legend, Norse stories and Plato's tale of Atlantis.
Adam