Phil wrote ...

Is it a brand new Speedo? If so it will keep exisitng mileage.

If it's a second hand unit, it's slightly more complicated. The mileage is stored both in the instrument panel, and in the BSI (the car's main computer)

If the used speedo has a lower mileage, it will correct itself to the mileage held in the BSI.

If it has a higher mileage, it will stay at the higher reading.