Source of speed and distance data?

I've used RWGPS for a couple of years with my iphones. I assume it has been inferring my speed by grabbing GPS data at intervals, and dividing the delta-distance by the sample interval. I've just thrown a PowerTap G3 hub on my bike and registered it with the RWGPS app.

On my test-ride, my ride results contain power, cadence, and distance. But the speed is stuck at -2.2mph.

When there is more than one source of data, how does the app determine which to use? I could have a speed/cadence pickup. My phone might have a barometer. How do I know (or set) where RWGPS has gotten its information?

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