Speedo Accuracy

[NIGHTHAWK]

How accurate are the speedos in our cars. I was in a friends toyota corrolla 97 model i think, when she was pulled up for doing 100ks in a 90 zone (qld). Of course he protested a bit to the officer that he was doing the correct speed.... but to no avail.
On the weekend we went on the highway and did 100ks side by side, and the only difference was the front of his car was up to my xfs front door. Surely the radar gun must have been out. Any one knows how to accurately check these things?