Originally Posted by
zacster
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.
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 think a speed sensor is able to better determine distance as it's not leaving those 1 sec. bread crumbs as does a GPS, nor is it susceptible to GPS signal dropouts. Thus it's always advised to use a speed sensor if mt. biking.