No the speedo don't have to be 100% I think it can be up to 8% either way so Mazda told us Wen we rang about the speedo being out in my misses 09 Mazda 3 its a load of crap if u ask. Me my vz wagon and rodeo are spot on why cant Mazda do yhe same job.
The law says a car speedo cannot read lower than your actual speed. Cars with speedos that read 8% out havent been sold in Australia for 40 years.
If your speedo were to be reading as 100 when you were in fact doing 105, the only possible explanation is your wheels and tyres, or a diff ratio change. If you get a speeding fine, it will be for one of those two reasons, or because the camera was faulty.
If both your wheels and diff ratio are original, then you may be innocent, and it may be the camera. Plead not guilty, dont spend any cash on lawyers, and hope the judge is in a good mood. He may give you the benefit of the doubt, especially since it could only be a few kmh over anyway.
No, its not worth spending money on or fighting too hard. The law is not on your side, a lawyer wont help. You just have to hope the judge shows some common sense.
EDIT: just an afterthought, since I dont own a VE.. for all I know the speedo is GPS controlled, I have no idea, in which case even gearing and tyres shouldnt make a difference. I dont believe that is the case though, but hey I could be wrong
It wouldnt surprise me to see that happen in the very near future though.. most new cars have a GPS installed anyway.