While planning a road trip spanning several countries I am having some doubts regarding estimates provided by Google Maps due to potentially poor roads. For example, this tour spans 13191 km and supposedly takes only 182 hours which gives an average of 72km/h. I've done some parts of this before where one can do 100 km/h or more in Ecuador and Peru but roads in Bolivia rarely could be driven past 50 km/h. I've not gone beyond Potosi before but since these roads pass through the Amazon jungle, I do not expect such velocity as I have taken other roads cutting through the Amazon before and could not exceed 30 km/h comfortably in most parts.
Does Google Maps know the actual speed that people drive on those roads or does it simply use the official speed limit? If the latter, how can we get a better estimate of reasonable speed to travel along a given road? I realize the actual speed driven depends on the vehicle and weather so I'm not looking for exact numbers just better estimates.