Hi all thought this might be useful if people are changing their speedo's. found this on Bimmerforums.com | The Ultimate BMW Forum.
To correct your mileage (copied and pasted btw):
You swap in your coding plug, and then run cluster test #9 to synchronize the cluster to whichever is higher - the cluster EEPROM or the coding plug - this cancels the tamper dot and shows the correct mileage for your coding plug. In my case, I swapped my 73,550 mile coding plug out of my US cluster and swapped it into my new euro cluster (frankenstein cluster because of the various pieces used to make it - 3.2L rear, US coding plug, 3.0L gauge faces, US plastic front piece) and ran that test.
You can find out how to do the tests here:
instrument cluster test