Originally Posted by Oldbear
That's how you change the diameter, but first you have to unlock the setting by unplugging one pin from the speedo.
On 2012 and later models it's the 18th pin (gray/brown). You can either cut the wire or take the pin out and insulate it.
After you've unlocked the speedo you can change the setting as described in the link.
Good point. I added that to the wiki. If anyone knows the details for pre-2011, I'll hunt for it but if you can add a pointer here, that'd be helpful too.