It's easy enough to get a few reference points of actual vs speedo speed and figure out if it's a linear ratio or a curve as mentioned above (where actual speed becomes speedo speed at 200kph).

I can pull up actual speed on my scanner or the hidden menu and compare at a few different indicated speeds. (I often describe some action I've done in my x5 at some speed and will usually say "I drove through this curve at an indicated 75"

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)