If I was an insurance company having to pay a claim. I could buy the GPS data, look at some anonymous GPS device that constantly goes to/fro the house of the person in the accident, followed by noticing that this person was speeding a few miles an hr and denying claims or claiming more responsibility, even if it is not warrented.
The flip side is that it can be a good thing. Funny thing about speed traps though... Guy gets pulled over for speeding 10mph above limit. Claims that hes moving with traffic (60mph). Gets ticket. 10 min later gets pulled over for creating traffic going 50mph, the speed limit, and gets off with a warning after showing the original ticket.
End of the day, this is very tricky, can be good and bad for society. However in the end OnStar is profiting so its not intended to help anyone but OnStar.
To apply the law equally, the driver can't provide a certificate of calibration for his GPS device so legally he's unable to prove that his GPS is giving an accurate speed measurement.
(As an aside, I don't think GPS would require any calibration anyway. If I understand it correctly, it'll pretty much either work or not work, with the accuracy of the output determined largely by atmospheric conditions and satellite geometry. The worry here would be deliberate tampering rather than calibration.)
But I agree that by far the more significant problem would be deliberate alterations to the data. It doesn't seem like it would be particularly hard to do so...
You're probably better off just paying the ticket.
Minor traffic offenses are civil, not criminal, so the much lower preponderence standard applies.