Towing capacities for a given model and configuration of a vehicle are determined by the manufacturer and are usually listed in the vehicle's manual or provided online at the manufacturer's website. These towing capacities are based on both the design characteristics of a vehicle and its performance in towing tests.
It is important to be sure of the exact equipment configuration of a particular vehicle when determining its towing capacity from reference material, as differences in equipment can have significant effects on the towing capacity of a car or truck. For example, the same model and year of a particular pickup truck can have very different towing capacities when using a standard hitch rather than a fifth-wheel hitch. Similarly, vehicles with four-wheel drive often have somewhat reduced towing capacity as compared to the same vehicle with two-wheel drive.