Quote Originally Posted by imdying View Post
Mmmmm... I do wonder. I know nothing on the topic, but I do know that everything has a tolerance factor. I wonder if there are any reviews on the net where people have measured different devices against each other...
There are a couple of fairly lengthy threads over at SBN on speedo accuracy, which include the same basic question. The consensus is that GPS, provided you are doing a constant speed on a flat straight, is bloody accurate. More accurate than a radar gun, which has a +/- 5% error, iirc. I think GPS is like 2% or less (it probably depends on how far you go and how constant you keep your speed).

In other words, you're never going to know exactly how fast you're going, but GPS is the closest to exact you can get. Obviously the more satellites you have linked in, the better.