Understanding the lingo

The towing terminology you need to know before your next road trip.

If you talk to any experienced caravanner or towing enthusiast, it can sometimes feel like they are speaking a completely different language.

They use terms like tare mass, payload and GTM, which may sound foreign at first but, with time, they’ll become second nature.

Follow this guide to understand the common towing lingo before you set off on your next road trip so you can tow safely and legally.

Trailer mass terminology

Aggregate Trailer Mass (ATM)

  • The combined weight of the trailer and its full load when it is not coupled to a tow vehicle.

Gross Trailer Mass (GTM)

  • The weight of the fully loaded trailer imposed on the trailer’s axle when it is coupled to the tow vehicle.

Tare Mass

  • The unloaded weight of the new trailer as delivered (including fitted options).


  • The trailer’s carrying capacity.  (ATM – TARE = Payload)

Tow Ball Mass

  • The weight imposed on the tow vehicle’s tow ball by the trailer or caravan.

Note: Tow ball mass (where provided) may be given as Ball Mass at TARE (this is of limited value as it is measured when the trailer is empty) or, Maximum Ball Load, which is the maximum the ball load can be for the tow vehicle.

Tow vehicle specifications

Towing capacity

  • The maximum weight the vehicle can legally tow (usually specified with and without trailer brakes).

Tow Ball Load

  • Tow Ball Load is the proportion of the trailer weight that is applied (vertically) to the rear of the tow vehicle. It will be listed as a weight in kilograms or as a percentage of the trailer’s ATM.

Gross Combination Vehicle Mass (GCVM)

  • GCVM is the maximum allowable weight of the trailer, tow vehicle and the load (including passengers, fuel and more) in the tow vehicle and trailer.

Tow vehicle Axle Load

  • A Maximum Axle Load specification is sometimes given for vehicles such as utilities, that are capable of carrying a load over the rear axle in addition to the load imposed by the trailer (the Ball Load).