KPH speedometers

The problem with just re-decaling the speedo is that while it may read speed in KPH, the odometer is still clicking off "miles"... I would think a more accurate way would be to figure out the % difference between KPH and MPH and regear it at the trans, or use a gear reduction/multiplier on the cable. That way the speedo & odo are both changed the same amount, and the existing numbers can remain on the face. ??