I don't trust those commercially available GPS receivers (especially if the chipset is a part of a cell phone). They're designed to give you a half-decent positional fix and some sort of a speed reading. Accuracy is not a priority. Most of them can get your location within 5-15m if you camp for a few minutes. Otherwise, YMMV (by A LOT). If you wanna have a so-so speed reading, you should maintain the same speed for a few minutes before you become slightly less non-confident as the reading is STILL gonna be off. I'd never use one to calibrate a speedo.
