You'd probably need to go to a speedo shop and get a certified test done - I've done that several times in the past. As for the ScanGauge accuracy, you must go into setup and specify the parameters such as engine size and most importantly tire size (via percentage from stock) - if you have gone from stock tires to say 31" tall, error is about a +9% difference that you can enter.

I was caught going 109MPH on New Year's day a few year's back and other than the 2 State Troopers approaching the VX with the gun's drawn, it was just a $250 ticket. Glad they only clocked me @ 109....