Changing wheel/tyre combination - rolling radius & speedo accuracy

[trapdoor]

Am thinking of getting a set of 19" wheels. Currently have 18's on which are all 235/50/18 but looking at a set of staggered wheels which are 255/50/19 and 285/45/19. Calculations suggest that there will be a speedo error of 6.6% under-read which will mean that a speed of 60mph I will actually be doing about 64mph.

Is there a setting so that the dealer can change the speedo calibration to take into account the different wheel/tyre sizes available for the X3 or is it that I HAVE to put on only the correct combination?

Thanks.