Originally Posted by
slawecki
GPS do not compute linear mileage, but use a delta and sum system. a point can be accurately determined with a gps, but the distance calculation getting from one point to another is not.
If I know my position with a given accuracy at any given time, I can calculate my speed within a certain tolerance.
Here's a study of GPS units on bicycles:
http://www.ncbi.nlm.nih.gov/pubmed/15519597
In there, the accuracy is typically within 0.1-0.4 m/s, or less than 1 mph.
With a WAAS-enabled GPS receiver, you can know your position with <1m error in most cases. If you update position every second, the maximum error is 2m in 2 seconds, or about 2mph. With a few seconds of averaging, that error will decrease.