Towing Terminology Guide – Caravan & Camping SA

Towing Terminology Guide

GVM (Gross Vehicle Mass)  

The GVM is the maximum allowable total mass of a fully loaded motor vehicle. This is the tare mass (mass of the vehicle) plus the load (including passengers). And includes any load or weight applied to the vehicle’s towbar.  

GCM (Gross Combination Mass)  

GCM is the rating provided by the tow vehicle manufacturer. The maximum laden mass of the vehicle combination (tow vehicle and caravan) is not permitted to exceed the GCM rating. Be aware, even if the tow vehicle and trailer individually are in their maximum rating, combined they may not be in the GCM rating 
of tow vehicle.  

ATM (Aggregate Trailer Mass)  

The total permissible weight of a trailer, which includes the weight supported by the tow vehicle (tow ball mass) and what you add as payload (e.g. water, gas, equipment and luggage). The ATM is specified by the trailer manufacturer and must not be exceeded.  

GTM (Gross Trailer Mass)  

The total permissible mass that can be supported by the wheels/tyres of the RV. This includes what you add 
as payload (e.g. water, gas, equipment and luggage), but does not include the weight supported by the tow vehicle (tow ball mass).  

Tare Mass  

The total weight of the RV (load on tyres plus coupling mass) with all options and fittings as supplied by the manufacturer, with empty water tanks, gas bottles, excluding fluids not essential for operation on public roads, and without luggage or personal effects.  

Tow Ball/ Coupling Mass  

The weight imposed on the tow vehicle’s towbar by the coupling. The tow ball coupling mass specified on a trailer’s Vehicle (VIN) Plate will be an actual weight recorded with the trailer at tare mass. The tow ball coupling mass will vary depending on the loading of the trailer.  


The allowable load carrying capacity or ‘payload’ is worked out by deducting the ‘Tare Mass’ from the ‘ATM’. It must not be exceeded under any circumstances. Safety, insurance, and warranty may be affected if the specified payload is exceeded.