The Australian Design Rules
"The Australian Design Rules require that a vehicle’s speedo must not indicate a speed less than the vehicle’s true speed or a speed greater than the vehicle’s true speed by an amount more than 10 per cent plus 4 km/h.
This means that the vehicle’s true speed must not be higher than the speed indicated by the speedo. That is, at a true vehicle speed of 100km/h, the speedo must read between 100km/h and 114km/h. An alternative way to look at it is; at an indicated speed of 100km/h, the vehicle’s true speed must be between 86 km/h and 100km/h. Generally speaking, on modern cars the true speed is only a few km/h less that the indicated speed.
GPS speed readings can be accurate, but not always. It is therefore recommended that you use your vehicle’s speedo for consistency."
My experience on highways (as recent as last Thursday) is that most semi-trailers including B doubles travel right on the upper speed limit of the GPS.... and try to push you along to either increase yr speed or get out of the way.
I normally travel at the posted limit minus 5kmh...so I try to get out of their way....