I don't believe your GPS'd speeds!

[Quattro 83]

Not really sure how much of an inaccuracy it causes, but Bushnell make a big deal out of it in the instructions.

"If the target is in a direct line (collision course) with the radar gun the measured speed will be exact. .....The measured speed will decrease as you move off this centre line. This phenomenen is called Cosine Effect, etc, etc.

As I said, not sure how much of an effect it has on Rc cars.