Originally Posted by
Notso_fastLane
The faster one goes (and more importantly, the faster one accelerates/decelerates) the more pronounced this effect becomes.
My favorite speed run around is about 1/2 mile straight with a 50 mph speed limit that I always exceed.
I typically hit speeds in the mid 60s as indicated by my wired wheel sensor, whereas the GPS (on a phone no less) rarely goes above 49-52. The wierd thing is, I can hold 65-68 mph for at least a couple yards of that run, but the GPS still lags enough that it 'smooths' out that top speed quite a bit. On my more normal routes, the smoothing is much less noticeable, usually within 3-4 mph peak.
GPS software performs smoothing on the raw speed data to mitigate suspicious jumps. Smoothing is basically a weighted average over a period of several seconds. Probably over a longer period if the sampling rate is longer - the Garmin "accurate" mode is once per second. Android GPS will vary, but the objective is to have the slowest sampling rate that has acceptable performance because high sampling rates cost battery power.
Short duration speed peaks will disappear due to the smoothing. Wheel sensor devices also do that, because the magnetic switches are noisier than you'd think with some random variation on exactly when in the cycle they activate. But they have a much higher sampling rate than do GPS applications, which is the main reason why that setup is inherently more precise than GPS.