Gross combined weight rating
The gross combined weight rating or gross combination weight rating (GCWR), also referred to as the gross combination mass (GCM), gross train weight (GTW), or maximum authorised mass (MAM), is the maximum allowable combined mass of a road vehicle, the passengers and cargo in the tow vehicle, plus the mass of the trailer and cargo in the trailer. This rating is set by the vehicle manufacturer.
The GCWR is a function of the torque output of the engine, the capacity and ratios of the transmission, the capacity of the driving axles and tires, the capacity of the radiator, and the ability of the chassis to withstand that powertrain torque.
GCWR primarily includes the powertrain's capabilities. Any of the powertrain's components, or combinations of, may create the weakest link in the powertrain. Mechanically, the differential gear ratio is not necessarily a weak link, but rather a ratio of power reduction or increase to the wheels depending on the ratio. A differential ratio of 3.42:1 will cause the engine and transmission to work harder than if the ratio was 4.10:1 with the same amount of weight hauled or towed. The tow vehicle will still move the weight regardless of the differential gear ratio. But a lower transmission gear may have to be used to move the weight, therefore increasing engine RPM, and fuel cost, and may increase the stress on the engine and/or transmission, therefore increasing the risk of premature wear and breakdowns. It may be noted that two identical vehicles with the only mechanical difference being the gear ratio will have different GCWRs and maximum towing capacities.