I am under the assumption that your GPS is:
1. Only as accurate as the amount of satellites it is accessing to provide your readings with.
2. Only as accurate as the money you sink into it. I am probably wrong

I have used mine for Car Travel on the dashboard and it seems to maintain the same speed reading as my Speedometer and that is going 70-75mph on I-95. So I would say that it is accurate.
You can find a good lesson on GPS's at How Stuff Works. Com GPS info