I would not rely on a hand held GPS to be very accurate with speed. I don't know why but for some reason if I run across the street with mine, held out in front, aimed up to get a good signal, it says I did 54 mph! I ran from the house to the beach one time, 50 yards, it said 35 mph! (no, I'm not that fast)
Do you really want to know ? Because the speed in a GPS is calculated by differentiating momentary travelled distance. This is very suspective to signal noise. This is also why all electroics and feedback systems try to transform "cirquits" so that only INTEGRATORS are included and no differentiators. Integrator are NOT so suspectable to signal noise. Now if you have a GPS with tracking ability you will find that the path and traveller distance recorded on your sprint WILL have been logged quite accurately. If you use this later in your PC software to calculate average speed THEN the read-out will be much more accurate.
Like I said the used method of acquiring a measurement can be and often will be very important in determining what accuracy you get.
So don't believe the GPS momentary speed read-outs as these are indeed very unreliable. Use the stored tracks and process these in the PC software and do get accurate measurements.
I have a much more expensive unit for aircraft (also by Garmin, like my handheld) that is very accurate on speed, it never puts out wild numbers, but it has a much better antenae system.
It is not the antenna system is the the implemented internal software that has been burned in a low energy consumption hardware chip. They don't use elaborate way of calculation here both for reasons of expense and energy consumption. Having said this ALL GPS units use about the same location hardware and internal software so these ARE accurate (within the specified 3 to 5 mtr accuracy)
So log the basic position data with a cheap handheld GPS (Garmin Geko 201) and use PC software to proces the raw data into speeds and travelled distances etc. You'll be surprised hpow accurate you can get.
Wouter
