Yeah its simple addition now really, no strange complicated rules.
The only thing that can fuck your speed is the amount of troops and equipment :if its really high it will lower your speed drastically, then if you move through forests/hills/mountains the slowing is multiplied by a lot and can lead to speed below 5 m/min even if you have enough horses.
So the only thing you have to be afraid of with caravans, is having too much troops/gear on it while going through difficult terrain.
The best caravans are around 500/1000 men with enough weapons for all and going through plains exclusively (except for really good shortcuts).