A vehicle's maximum towing weight is often assumed to be a measure of engine strength. It is actually the lowest figure permitted by several separate limits, and the engine is rarely the one that decides it.
The weights that define the limits
Several figures apply at once. Kerb weight is the vehicle empty, gross vehicle weight is the maximum with occupants and load, and gross combination weight covers vehicle and trailer together.
A towing limit that appears generous can be unusable if the combination weight does not also allow for passengers and luggage.
This is why a fully loaded vehicle may legally tow considerably less than the headline number suggests.
Why braking usually sets the ceiling
Adding a trailer roughly doubles the mass that has to be stopped while the vehicle's own brakes remain the same size.
Braked trailers carry their own brakes, which is why the permitted braked and unbraked figures differ so sharply, often by a factor of several.
Thermal capacity matters as much as outright force, since repeated braking on a descent heats the discs and reduces what they can do.
What cooling contributes
Towing places the powertrain under sustained high load, frequently at low road speed where airflow through the radiator is weakest.
Transmission fluid temperature is often the binding constraint, because an automatic gearbox working hard against a heavy load generates considerable heat in its torque converter.
Manufacturers add cooling capacity to towing packages for precisely this reason, and the rating assumes that equipment is fitted.
Why stability limits the geometry
A trailer behind the rear axle acts as a lever, and downward force at the towball transfers load off the front wheels and onto the rear.
Losing front axle load reduces steering and braking authority, which is why a maximum noseweight is specified alongside the towing limit.
Wheelbase and rear overhang determine how much leverage the trailer has, which is why long, heavy vehicles tow more stably than short ones of similar power.
Why ratings differ between markets
Manufacturers rate the same vehicle differently in different regions because the test procedures, legal frameworks and expected use are not the same.
Some markets require a demonstrated hill start with a fully laden combination, and some specify sustained speed on a defined gradient.
The result is that a rating is a statement about a defined test and a defined configuration, not a universal property of the vehicle.