Originally Posted by
bikeguyinvenice
Curious as to which is more accurate for recording distance and speed and time. I have been using the cyclemeter app for iphone, which I have noticed sometimes has hiccups in the recorded speed, elevation and time. On todays ride the cyclecomputer recorded an average speed of 14.1 mph over 29.36 miles and my app recorded and average speed of 17.8 mph over 29.29 miles.
What cyclecomputer?
The cycle computer and the phone probably report the
total time (including stops) and the
moving time. You should be able to determine which time was used to compute the average (given that the distances are the same).
NOTE: different computers might use different low speeds above (or stop times below) which the computer thinks you are moving.
By default (i.e., without a wheel sensor), the Garmins use GPS for speed, which can be inaccurate over short periods of time and distance. The 500 and up units allow you to use a wheel-rotation sensor that avoids the GPS inaccuracy.
GPS (basically) just determines location/position (and assigns a time to the location). Speed is determined from the position data by computing the distance between two points and dividing by the difference in time. Since GPS positioning has some error (+-30 feet or so), the speed between determined from points close together will be less accurate (more variable). GPS speed works better when traveling at higher speeds because the points being used are farther apart.
Speed from wheel rotation is very fast and works accurately at distances much less than the normal GPS position error.