You cannot rely on the accuracy of GPS to keep you out of jail. There are a lot of variables and your car is moving fast.
The legal requirement for speedometers (regulation) is:
The regulation states that, when it comes to the acceptable margin of error for speedometers, that speedometers must never under-report a vehicle's speed, while it must never over-report by more than 110% of the actual speed + 6.25mph.
I think this means your speedo could be indicating 82.5mph when you are at 70mph.
I have a head up display that reads speed from the ODI II port. It does not always agree with the cars speedo.
If you use your cars' speedo and drive the limit you should not get a ticket...........