Gross combined weight rating

The gross combination weight rating (also Gross Combination Mass and maximum authorised mass), which can be abbreviated to GCM, MAM, GCWR is the maximum allowable combined mass of a towing road vehicle, 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.

When used with trains in the UK, the GCWR is also known as the gross train weight (GTW). The GCM is a function of the torque output of the engine, the capacity and ratios of the transmission, the capacity of the driving axles and tyres, 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 move the weight, therefore increasing engine RPM, and fuel cost, and may increase additional 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.

See also

• Curb weight
• Heavy-duty vehicle
• Light-duty vehicle
• Vehicle category
• Gross axle weight rating (GAWR)
• Gross trailer weight rating (GTWR)
• Gross vehicle weight rating (GVWR)
• Corporate Average Fuel Economy (CAFE)
• Truck classification

References

A Lesson on Gross Combined Weight Rating