Basic Towing Guide & Definitions

Posted by Motorama in Buyer Advice

Getting your vehicle set up for towing can help you out whether you need to move house, get out on the water in a boat or a jet ski or set up a holiday adventure on the road in a caravan.

Learn about all the terms you need before you get hitched.


The Gross Combination Mass (GCM), sometimes called the Gross Combination Vehicle Mass, refers to the maximum weight allowed on the road of both your car and the trailer, with everything on board inside the car, including passengers, cargo and fuel – as well as the full weight of the trailer, boat or caravan hitched to the back.


Gross trailer mass (GTM) is what you can safely and legally load into the trailer as payload, and can be supported by the wheels. This includes water, gas and luggage but does NOT include the mass supported by the tow ball.


Aggregate Trailer Mass (ATM) refers to the tow ball mass, as well as the payload of the trailer when fully loaded. The manufacturer of the trailer will make this information available, and it’s not safe or legal to exceed this weight.

Tare Mass or Tare Weight

The tare weight refers to the empty weight of the trailer or, in the case of caravans or boats, with no modifications or extra equipment.


The payload is the difference between the tare mass and ATM, and refers to the carrying capacity of the trailer while travelling. This usually is no problem, but because some caravans already have a limited payload – any modifications such as additional water or gas storage may exceed the safe and/or legal payload limit very quickly.

It’s important to remember, if you’re in an accident or suffer a breakdown, your insurer or warranty provider (for either the caravan or the tow vehicle) may not pay a claim where the payload was exceeded. Besides, it’s not good for your vehicles fuel economy or on-road performance to push the limits of the payload of either your trailer or the towing vehicle.

Tow Ball Mass (TBM)

The tow ball mass (TBM) is the difference between the GTM and ATM, and refers to the downward weight on the vehicle’s coupling. This weight was not normally advertised on older caravans or trailers, but newer models should include the figure. The TBM is also included in the tow vehicle’s payload when your caravan or trailer is attached.

If you’re looking for your next tow vehicle, the team at Motorama can show you the best new car to load up your caravan, as well as over 800 used cars. And if you’re already to get on the road, make sure you book in for a service at any one of our service centres in Moorooka, Browns Plains or Springwood and let our expert service technicians ensure that your car is ready to tow anything you need.