There's the 3 answers which are a nice even mix of convenience and price.
The cheapest solution is to change the gear, this should get you close but may not be exact.
Modifying the eeprom would allow you to get it exact but means you would be without your speedo for a few days.
Dave's solution would allow you to get it exact and you don't have to send your speedo away, plus you can fiddle with it if you want in case you got it wrong the first time
EDIT: Just to add Daves solution will allow you to compensate for tyre wear to as you can have a fiddle when you notice the reading has changed