The speedo and mileometer both work off the wheel sensor which in my case is I think on the OSF wheel. Although I haven't had them fitted yet, I expect my new Blockleys to change the speedo reading slightly, on account of their having what looks like ever-so-slightly taller sidewalls, not by change in size spec, but by the more squared-off design. If these have a very slightly taller sidewall, the speedo will become MORE accurate and will 'eat into' some of the +5-10% over-reading tolerance.
