wondering how many of us are using gps speedos

[Krait]

On level roads GPS is quite accurate if your unit is getting a good satellite signal. I trust mine nearly to the km and it hasn't failed me yet. One thing I do is establish the inaccuracy of the standard speedo so that even without the GPS I'm aware of the margins of error. I know that 120kmh indicated on my road car is slightly less and therefore safe, hasn't failed me yet.

GPS speeds on hills are another matter as they can be inaccurate due to the lack of barometric data. Generally a GPS is only designed to measure distance vs time which when applied horizontally is fine. On a hill the GPS will continue to measure speed horizontally and ignore any vertical change of position. Therefore the GPS will indicate a slower speed on hills than the car is actually travelling.

I'd stick with a standard electronic speedo and use a GPS to calibrate it on a straight and level road.