How fast can a Donkey pull a cart both empty and filled with grapes (or any cargo) per round?
20' per round. 2 miles per hour and 16 miles in a day.
Rules listed below.
Transport, Land, Cart
This two-wheeled vehicle can be drawn by a single horse (or other beast of burden). It comes with a harness.
A cart is open-topped. The most common dray creature for a cart is a mule, which can pull the vehicle at a speed of 20 feet even if it’s fully loaded.
The prices listed are to purchase the vehicle. These prices generally exclude crew or animals.
Empty Weight: 200 lb. Capacity: 1/2 ton
Where are you getting the 20 ft. per round from?
Where are you getting the 20 ft. per round from?
Page 174. It does not list a move speed of 20', only the 2 miles per hour. But 2 miles per hour works out to 20' using the PFRPG table for general movement on page 172.