oooh, i know about this topic, madness
well depends WHY your speedo is out guys, and how much ...
im unsure about how the elecrtical speedo in skylines work, but usually you either replace the gear inside the gearbox that sends the signal to one with fewer or more teeth, to get it correct.
try, automotive instrument repairs in mulgrave, just off springvale road near the 5 way junction.
95612366
I've had an old car of mine calibrated there, friendly bunch of people... There is also another mob, but they are up in the northern subs