The speedo cable output on the Alfa box is driven by the output side of the differential, so is fixed to the wheels rotation. If the rolling diameter of the wheel/tyre combo you are using is the same as the original Alfa fitment, then the speedo output will be correct. Mine appears accurate with my 578mm diameter 886 revs per mile tyres.
Original Alfa seems to be a 195/65 x 15" giving a 635 diameter which should make mine over read significantly!
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