Changing clock's

[Alan Gunn]

I have a problem with my speedo sticking and not dropping back to 0
What i want to know is if i fit a second hand set of clock's will my mileage stay at what it was or will it read what the reading was on the car they came off.
If they stay at the doner car reading can i reset the mileage to what the old clock's had on them.
Thank's
Al