Written byGross trailer mass (GTM) is the maximum weight a loaded trailer or caravan may place on its own axles while hitched to the tow vehicle, set by the trailer manufacturer.
Also known as: GTM, gross trailer mass, GTM rating, trailer GTM, caravan GTM
Key points
- GTM is the trailer's loaded maximum minus the tow ball weight, which the tow vehicle carries once the trailer is hitched.
- Tow ball weight counts against the tow vehicle's GVM, and the vehicle and trailer together must stay within the GCM.
- Brake rules follow GTM: above 750 kg a trailer needs brakes, and above two tonnes every wheel must be braked with a breakaway system.
- A weighbridge reads GTM when the hitched trailer's wheels alone are on the plate, and ATM when it is weighed uncoupled.
How GTM works
Every trailer carries a compliance plate listing its tare weight, its aggregate trailer mass and its gross trailer mass. The first is the empty weight, the second the maximum the whole trailer may weigh standing alone, and the third the maximum its axles may carry once the coupling is on the tow ball. The gap between the second and third figures is the tow ball weight the manufacturer designed the trailer around, typically a tenth of the loaded weight.
GTM is the limit the axles, suspension, wheel bearings and tyres are rated for, which is why it governs the brake rules and why a trailer loaded tail-heavy can be under its total limit yet still overload its axles. Tyre load ratings on a caravan are chosen to cover the GTM with a margin, and an inspection of a used van includes checking that the tyres fitted still do.
GTM, brakes and towing capacity
The Australian Design Rules use GTM to decide what brakes a trailer must have. Up to 750 kg none are required. Above that, at least one axle must be braked, and above 2,000 kg every wheel must be braked and a breakaway system must apply the brakes if the trailer detaches. Electric brakes with a breakaway unit are how most caravans meet that rule.
Towing capacity is a separate check, and GTM plays no part in it. A tow vehicle's braked towing rating is compared with the trailer's loaded weight, or in practice its plated ATM, and its tow ball rating with the actual ball weight. Both sit inside the vehicle's limits: the ball weight uses part of the GVM and rear axle capacity, and the vehicle plus trailer must sit within the GCM. A trailer under its GTM can still put a loaded ute over its GVM.
GTM when buying or financing a caravan
Lenders identify a caravan or trailer by its VIN and do not check whether the buyer's car can tow it, so the weights are the buyer's homework. Reading the plate before signing avoids financing a van that cannot legally leave the driveway behind the family SUV, and it feeds the total cost of ownership because a heavier van may mean a new tow vehicle, a brake controller and a weight distribution hitch.
Insurers ask the same questions after an accident. An overloaded trailer, or one towed beyond the vehicle's rating, gives an insurer grounds to refuse a claim, and exceeding a plated limit is an offence for the driver, whoever owns the van.
Example
A dairy farming couple from Echuca collect their new caravan and take it straight to a weighbridge before the first trip, with water tanks full and the annexe, chairs and bikes loaded. Hitched to their wagon, with only the van's wheels on the plate, the reading sits just under the GTM on the compliance plate. Unhitched, the total is over the aggregate trailer mass, because the tow ball weight is higher than the brochure suggested. They leave the bikes at home, run one water tank down to half and weigh again, and both figures come in with a margin. The receipt from the weighbridge goes in the glovebox with the insurance papers.
Not to be confused with
- Aggregate trailer mass (ATM)
- aggregate trailer mass is the whole loaded trailer standing alone, while GTM is the weight on its axles once hitched
- Gross vehicle mass (GVM)
- GVM caps the tow vehicle, including the tow ball weight it carries, while GTM caps the trailer's axles
Frequently asked questions
What is the difference between GTM and ATM?
Aggregate trailer mass is the maximum the loaded trailer may weigh when it is uncoupled and standing on its own wheels and jockey wheel. Gross trailer mass is the maximum on its axles once it is hitched, which is the ATM less the tow ball weight the tow vehicle then carries.
How do I measure GTM?
Hitch the loaded trailer to the tow vehicle and drive onto a public weighbridge so that only the trailer's wheels are on the plate. That reading is the GTM as loaded. Weighing the trailer uncoupled gives the aggregate trailer mass instead, and the difference between the two is the tow ball weight.
Does GTM decide whether a trailer needs brakes?
Yes. Under the Australian Design Rules a trailer up to 750 kg GTM needs no brakes, one above 750 kg needs brakes on at least one axle, and one above 2,000 kg needs brakes on all wheels and a breakaway system. Most caravans fall into the last group and rely on electric brakes.
Is towing capacity compared with GTM or ATM?
ATM, or the trailer's actual loaded weight, never GTM. The ball weight portion of the load is carried by the tow vehicle and counted against its GVM and tow ball rating instead. Dealers and insurers usually check the plated ATM against the towing capacity, and the vehicle plus trailer must also fit within the GCM.
Related terms
Gross vehicle mass (GVM)
Gross vehicle mass (GVM) is the maximum a vehicle can legally weigh when fully loaded, including fuel, passengers, accessories and cargo, as set by the manufacturer.
Read definitionGross combination mass (GCM)
Gross combination mass (GCM) is the maximum a vehicle and everything it is towing can legally weigh together, as set by the vehicle's manufacturer.
Read definitionAggregate trailer mass (ATM)
Aggregate trailer mass (ATM) is the maximum a fully loaded trailer or caravan may weigh when uncoupled, tow ball weight included, as set by its manufacturer.
Read definitionVehicle identification number (VIN)
A vehicle identification number (VIN) is the unique 17-character code a manufacturer stamps on a vehicle, used to identify it on registration, insurance and finance records.
Read definitionInspection
An inspection is a structured check of a leased or financed asset's identity, condition and usage against the contract, done before delivery, during the term or at return.
Read definitionTotal cost of ownership (TCO)
Total cost of ownership (TCO) is the full cost of buying, financing, running and disposing of an asset over a set period, not just its purchase price.
Read definitionGo deeper
Sources
This article is general information only and is not financial advice.