How does the OBD200 calculate mileage?

OBD200 mileage

If the vehicle supports the OBDII Odometer PID, the modules will use that meter reading. If the vehicle does not support the PID Odometer, and most of them do not, then it uses the DTC (Distance since Trouble Codes cleared) as the meter.

In either case, using the MR55, the user programs a starting odometer into the OBD200 module (what's showing on the dashboard odometer, usually), and as the selected mileage increments, the module odometer increments at the same rate.