Having both GPS and ANT+ speed sensor (Bontrager) give very inaccurate instant speed datas.
In fact the speed is constantky jumping from one value to another and most of time by jumps of more than 10 to 15 kiloùmetres par hour.
With only the GPS runing, speed vary constantly but with a implementation of few km/h wich is more realistic.
In fact on my road bike I am not using the ANt+ sensor anymore.
I, therefore continue to use it on my MTB due to lack og GPS signal from time to time.
Whe both are used together how does it work? GPS extrapolationg the wheel size? There is no set up procedure to do as for a ffot pod.