I have noticed that my on water Speedsurfing data as displayed on the Coros watch and mobile app, is understated vs the result as calculated by GPSspeedsailing processing a .gpx.

I have reviewed the data sec by sec and the GPSspeedsailing processing is correct.

for example today on water the Coros Speedsurfing app reported a 2 sec max 30.79 knots

vs GPSspeedsailing 2 sec max 33.29 knots

Any ideas / reasons.


Hi Craig,

The reason is simple: the .GPX files are only the data points. No Doppler, no Kalman corrections. The .FIT file is the only thing you need: 100% Doppler and it has all the necessary data fields. So, forget about the .GPX files in Coros for speed surfing (if you plan a bike route .GPX is usefull). In GPS Results you can load the .FIT files and make your usual analytics. PS: already using the auto upload to

thank you. 100% correct.- Coros Speeding app is reporting "record' alert and data max's perfectly in real time - all line up with .fit file as processed by GPSSpeedsurfing. I have linked Coros for auto upload to for future session. Thanks again.

Hi Craig, 

Good to hear it's fine now. I made a small manual for Coros Speedsurfing mode, if you want I it please sent an email to

Hi Craig,

I have the same problem and will move to .FIT now. However, two days ago, I used a Motion GPS which gave me an average of the 5 best 500 m runs nearly 1 kn faster than the faster 500m measured by Coros. Not very convincing, it seems.  Thanks for any idea.


Hi Craig,


Can you please sent the 2 track-files to us via We have seen it more often the the motion is a bit more positive, partly caused by the logging frequency difference and some unkonws but this difference is huge and we would like to learn from it.  



Hi Dylan, unfortunately, I do not have the logs. The organizers of a speed competition provided the motion devices and photographed the final results which deviated from my watch as described above.

I will try to do some proper tests once I received a motion...