I compared Wahoo Fitness using GPS for speed to my friends using dedicated garmin with wheel sensor and the difference is negligible when riding. The GPS takes a couple seconds more to adjust for increase/decrease in speed. We also compared distance logged at the end of the ride and again its negligible in the big scheme. I know wahoo lets you set the wheel circumference if you're using a sensor. I would guess the model phone/OS is the blame if there's big reading descrepencies if using popular apps like STRAVA, WAHOO, RIDEWGPS, etc. regardless of using GPS or sesnsors. Those apps are popular for a reason.