Ant+ speed sensor limitations ?

[sbouju]

I have tried an Ant+ speed sensor in Locus Map in different configurations (compared with a GPS speed stabilized measure), and founded some limitations/errors in its results:
- a poor precision at low speed (under 10-15 km/h)
- false indication when reaching high speeds (over 75-80 km/h), with the indicated speed not growing more.

Any idea of the reasons of that ?

Anyway, the measures seems relatively correct between 15 and 70 km/h
https://www.youtube.com/watch?v=5F3GFEbdVwo

At low speed in the first half of this under second video:
https://www.youtube.com/watch?v=CmhgXX_T4pA

The sensor I test is this one:




Note: not able to mount it on the hub of the wheel as figured above (it is with my EUC or my car), I mount it on the side of the rim... May be a problem ?