A question in our mailbox. "According to my bike computer, I rode a ride with an average speed of 30.0 kph. However, when I upload this ride to Strava, the average suddenly drops to 29.9km/h. Also, the total distance deviates slightly. How come?" Let's dive in.
Strava always uses GPS data to calculate average and total distance, even if you use a speed sensor. One of the reasons Strava does this is that a speed sensor can give false readings if you set the wheel circumference incorrectly. So you could also very easily cheat with it. In principle, however, a speed sensor is more accurate, provided the wheel circumference is set correctly (with the most modern sensors this is automatic).
A bike computer, on the other hand, uses the speed sensor for average speed and total distance, provided there's one. In that case there can be a small difference in the total distance, and thus in the average. A GPS always suffers from drift. You can also suffer from e.g. tunnels or foliage. If you choose the option "correct distance" in Strava (see image) an algorithm will remove most errors from the GPS data ("corrected Strava distance"). This option also works if you measured with a sensor, by the way. Strava then uses the GPS data and ignores data collected with a speed sensor.
The second reason is how breaks are handled. Most cycle computers have an auto start/stop function. You can often set the threshold value yourself, the default value is 5 kph. Values below that are in the FIT file but are not included in the calculation of the average. Strava, however, makes its own calculation of stops. They do this, as we explained above, based on GPS data. One problem, however, is that a GPS signal is notoriously unreliable at low speeds. It's quite normal to sometimes see 0.5 kph on your head unit while standing at the stoplight. If you're riding without a speed sensor, you'll also notice that at the traffic lights the car start/stop switches on much less quickly than when you're using a sensor. Speed via GPS has a tendency to wobble quite a bit.
To solve this, Strava uses a threshold value, basically meaning that the first few seconds of a stop don't count, i.e. the time just continues to run. Strava is therefore a bit stricter than most cycling computers, and that's why the average often ends up a few tenths lower. The more you stop, the bigger the difference.Thanks to an Ilja Booij and Michiel van Lonkhuyzen.