[Gaz64]

Some more info for discussion:

The big change with ADR 18/03 means that the speed indicated must always be ABOVE actual speed. Prior to 1 July 2006, a speedo could read over or under the vehicle actual speed - as long as it was within 10% of the actual speed. The driver/rider can only rely on their instrument, their speedo. Owners of older vehicles (made up until July 2006) are disadvantaged. ADR 18/03 requires speedos to overestimate the actual speed of the vehicle.

A Possible Scenario
Lets consider for a minute the end result of different ages of vehicles travelling down the road sign posted as 60km/h. This segment of road will have a varied population:
Those aware of the indicated speedo error on their post 2006 speedos, deliberately driving at the speed limit as set by indicated speed (comforted in the knowledge that they'll never get a speeding fine),
Those aware of the indicated speedo error on their post 2006 speedos, adding a fudge factor to their indicated speed,
Those not aware of the indicated speedo error in their older pre-2006 speedo taking their indicated speed at face value and driving at the speed limit (may be 10% over limit),
and finally those with GPS's who will be driving around at a true actual speed.
If they are all doing an indicated 60km/h, so complying to the prevailing speed limit, their actual vehicle speeds could be anywhere between 49km/h to 66km/h. That's a big differential to manage.

Now add to that mix, those in the 3rd category driving at an indicated 5km/h under the speed limit as encouraged by the likes of the Victoria TAC i.e., the 'wipe off 5' campaign, and the differential in actual speed in a 60km/h zone could be as wide as 44km/h to 66km/h. No wonder there's road rage.

The next time you get agitated by a vehicle travelling just under the limit, have some consideration, because as far as they know, they may think they are doing the speed limit.

Conclusion
"It was pointed out to me by a solicitor that another change made in 2006 was to clarify that the onus for a vehicle's speed is firmly on the operator, regardless of how they determine it. The excuse of having an inaccurate speedometer is now irrelevant in law as it is the responsibility of the vehicle operator to ensure it is accurate or that they at all times are knowing of their velocity be it by GPS or ESP or some God given gift."
Grant Delahoy - VMC

Reference Material and Websites
The MRASA would like to recognise the following sources and contributors of information on the subject of speedometer accuracy.

Traffic Law Website - a Victorian website with a law-centric view of speedometers.
(Authority to quote and reproduce obtained from Sean Hardy, 12 March 2014)
Guy Stanford, Australian Motorcycle Council (AMC) - material from the Australian Design Rules.
Grant Delahoy, Victorian Motorcycle Council (VMC)
Rob Salvatore, Victorian Motorcycle Council (VMC)

My point is GPS may well be accurate but it should NOT be the only source of driver speed info. Doesn't work in tunnels, covered carparks etc, or within large buildings in the city etc.

Relying on an external source to the vehicle is not a fixture of the vehicle.

Gary