If anything I would think that the speed display would use the sensor when available, but miles would use the GPS as it is more accurate over the long term. That said, my old Cateye Astrale 8 wired computer and RidewithGPS on my phone always have the same miles/tenths on any trip, and speed is always more accurate on the Cateye, at least until I paired it with a sensor just this past week and these are always equal now too.

Even though I changed all the cables and bar tape on my road bike just yesterday, and already have Wahoo sensors that pair with RidewithGPS, I re-installed the Cateye. Something about my phone running out of power every time I use it on a long ride even if I leave the screen off...

I'm still considering the Bolt though. I already have the sensors anyway, plus an older Garmin HRM. I could replace both the phone and the cateye with it. I could use it on my commuter too, just to record miles since I don't really care about much else when riding that bike. Cool that it comes with two mounts.