What car is it, F or TF?
If it is LCD mileage display, then the actual value is both stored in the speedo
and at least one of the control units in the car (cant remember which one, possibly the alarm or ECU).
When you replace the speedo then the highest stored mileage is transferred to the mileage (or to the Alarm module), so you should get a correct indication.
I changed the ECU and alarm module due to a fault, and gained 5000 miles
I have a 5000 mile insurance policy