Since 1982 - speedos have been legislated to NOT indicate lower than ACTUAL VELOCITY! Before this, it was a manufacturer's 'free for all'. Since then they MUST INDICATE HIGHER VELOCITIES, and are not allowed to show under.
Generally speaking - you should be able to safely (read LEGALLY) calculate a 7% difference - i.e. 107km indicated = 100 actual - BUT......
Some states are lowering the tolerances for applicable fines, in that you could get away with up to 9km/h over the speed limit years ago, now you would get booked for this indiscretion - spesh in Qld and Vic.
So don't push the envelope as you will get NO QUARTER!!! This is even worse for the States that have farmed out certain speed detection offences to private companies where they earn $$$ by fining you - NOT SHOWING LENIENCY!
Be careful out there Villains!
Cheers, Diesel.