Right, so, by a process of deduction, to get km/h on the Alternate Speed Display, you need to set your Units to a US setting - either "mpg (US), mi" or "mpg (GB), mi" - and then the primary digital speed will be mph, and the alternate in km/h. Pity that's then tied to mpg for your fuel readout.
I'm happy with my speed display as a separate MFD page, personally.
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