FYI, GPS is only accurate if you are travelling on the flat, as the gps satellite network can only determine your location in 2 dimensions.
Eg. If you were travelling up a 45 degree slope at 100km/h GPS would report only your horizontal speed, which in this case would be 50km/h.
Something to consider before thinking of gps speed as an authoritative source.
Many GPS units have altimeters actually, which allows them to determine altitude (I suspect including one is a cheap addition that allows them to be more accurate) and all phones do.