As I have ranted earlier in the Data Recording interval thread, my Edge 25 is consistency inaccurate in calculating the distance travelled just from GPS data measurements. I was wondering if pairing a speed sensor would make any difference to the trip distance computation or will it only make the speed measurement more accurate?
For normal road biking a wheel sensor won't improve the overall accuracy of the ride data. GPS data has position errors. But those errors cancel out over distance so, I think, total distance is about as accurate as you can get. The position errors can have short term effects that cause the gradient readout to be less accurate, auto-pause to come on when moving slowly and inaccuracies in the displayed speed.
In areas of poor GPS reception like mountain biking the sensor will improve overall accuracy.
FWIW I use it the device only for road biking and commutes so poor GPS signal is not an issue. The main problem is the lack of one second recording which makes the total distance travelled less when you are not going in a straight line. If despite having the speed sensor the Edge 25 still only records once in 4 seconds, then I don't think having a sensor will make any difference to the accuracy.
Well if Strava is what worries you, then no, using a speed sensor won't help. The reason is that Strava always recomputes the distance from the GPS track and always ignores whatever the device puts in the FIT file as the total distance. Garmin Connect and many other places do the opposite, always use the device total distance and ignore the GPS track, and some other places/software give you a choice. So, if the GPS track were perfect, Strava would always short the distance traveled, since, although they have access to maps, I don't think they compute the "along the road" distance between two points, only the straight line distance. Of course, since the GPS track has some error, it isn't clear if it will always be shorter. My mountain bike ride yesterday (not particularly challenging for GPS, although the forest is now fully leafed) shows 19.0 miles on Strava and 19.08 on Garmin Connect. The FIT is from an Edge 1000 using 1 s recording and a speed sensor. A speed sensor has absolutely no effect on the recorded GPS track, only on what the device displays and records for speed/distance (the total distance in the end is the total number of wheel rotations x circumference of the tire).
I wouldn't worry much about it, though. Distance by itself is not a terribly useful measure of cycling activity (consider the difference in effort between a ride on the same route on a cold, windy day and a warm windless one, or a ride with lots of climbing vs a flat ride).