Originally Posted by
cafzali
If you don't have a cadence meter, there will be a data lag because it takes time for the info to reach from the satellite to you.
??? No, that's not the reason for the variation in speed.
Originally Posted by
Weatherby
I took my Garmin 800 out of a maiden voyage tonight and the speed display fluctuates wildly (jumps around by 1-2 mph) when in fact my speed was not changing as indicated by the seat of my pants and my cyclecomputer.
You don't really understand the tool.
Garmin | What is GPS?
GPS.gov: GPS Accuracy
There is variability in the position measurement of GPS receivers. Garmin says +-15 m, the FAA says +-3 m for "high quality" (large, expensive) GPS units. (The variability can be reduced if the device can WAAS or differential GPS.)
Given the
unavoidable variation, it's expected that the speed will "jump around" a bit.
A 1-2 mph speed is a change of 0.447-0.894 m per second (much less than the variation expected).
(At high rates of speed, like in a car or airplane, the GPS determined speed is very accurate.)