When the speedo is converted from Kms to Miles a certificate of conversion is issued with the number of Kms when the conversion was carried out and the issuers name. After this point the odometer racks up in Miles.
to find the true mileage of the car, take the odometer reading, deduct the value on the certificate from it - that is the number of miles the car has covered since conversion. Multiply the value on the certificate by 0.621 and add it to the first value - presto