Victorian Speeding Laws vs Speedo Accuracy

Ok, so I was doing a little bit of reading after getting a bit sick of the revenue-raising argument a lot of riders put up on forums (thankfully none here!), when I came across this: http://www.camerassavelives.vic.gov.au/home/cameras/fines+and+penalties/
Which states a +/- tolerance of 2km/h for fixed camers and 3km/h for hand held devices. This sounded a bit strange to me, as I thought spedometer dials in australian cars at least were not so accurate.

A quick bounce to wikipedia (and the CITED line) showed that in fact, "There were no design rules in place for speedometers in Australia prior to July 1988. They had to be introduced when speed cameras were first used. This means there are no legally accurate speedometers for these older vehicles. All vehicles manufactured on or after 1 July 2007, and all models of vehicle introduced on or after 1 July 2006, must conform to UNECE Regulation 39.[7]

The speedometers in vehicles manufactured before these dates but after 1 July 1995 (or 1 January 1995 for forward control passenger vehicles and off-road passenger vehicles) must conform to the previous Australian design rule. This specifies that they need only display the speed to an accuracy of +/- 10% at speeds above 40 km/h, and there is no specified accuracy at all for speeds below 40 km/h. All vehicles manufactured in Australia or imported for supply to the Australian market must comply with the Australian Design Rules." The [7] Citation refers to the UNECE Regulation.

Now, I don't speed unless it is nessisary to escape imminant danger. I've been tempted once or twice, but I've never intentionally raced off. Also, I don't think it's really appropriate to do, and though the cops MIGHT be out revenue raising, if you get caught doing it you were still breaking the law.

HOWEVER
A +/- 10% at 40 km/h is still a kilometer over the most lenient sensitivities. If you're doing 60, a brand new speedo could show you anywhere between 54 and 66. An enormous margin.
I used to study science at uni, covering laboritory experiments and accuracy testing across a wide range of disciplines. I don't see how the government can realistically defend a law that allows an individual to be incapable of preventing themselves from knowing they are breaking it. I know ignorance is no excuse, however I would think this goes a little beyond that...?

Comments? queries? Should we all drive at 10% less than the signposted limit? That's sure to piss off other road users... doing 55 in a 60 zone will annoy them, a motorcycle on the freeway doing 90... they'll probably just run you over.

And I'd LOVE to see what would happen if you calmly, politely and patiently explained to the police officer that you couldn't know what speed you were doing exactly because of the inherint inaccuracies of your speedo.

Actually, I'd like to chat to a traffic officer reguarding this (not while being served with an infringement).
Do they get annoyed if you pull over to ask them something?