A tow vehicle and a camper trailer are governed by four rated masses at once, and only one of them binds on any given day. On a well-equipped dual-cab towing a mid-weight camper, the one that binds is almost always the tow vehicle’s GVM — while the gross combination mass, the number owners most often check, still shows hundreds of kilograms of apparent headroom. That headroom is not payload. It cannot be spent.
This explainer sets out what the Australian Design Rules actually say about the four masses, shows the arithmetic that connects them, and works one clearly labelled hypothetical example through to the point of failure. Every definition quoted comes from the Vehicle Standard (Australian Design Rule – Definitions and Vehicle Categories) 2005 as compiled on 6 December 2025 (Compilation No. 16). No real vehicle or trailer is described, and no manufacturer’s figures were used.
The four ratings, as the Australian Design Rules define them
| Rating | ADR definition (quoted) | What it constrains |
|---|---|---|
| GVM — Gross Vehicle Mass | “the maximum laden mass of a motor vehicle as specified by the ‘Manufacturer’” | The tow vehicle alone, fully loaded, including the load coming through the towbar |
| GCM — Gross Combination Mass | “value specified for the vehicle by the ‘Manufacturer’ as being the maximum of the sum of the ‘Gross Vehicle Mass’ of the drawing vehicle plus the sum of the ‘Axle Loads’ of any vehicle capable of being drawn as a trailer” | The whole combination |
| ATM — Aggregate Trailer Mass | “the total mass of the laden trailer when carrying the maximum load recommended by the ‘Manufacturer’. This will include any mass imposed onto the drawing vehicle when the ‘Combination Vehicle’ is resting on a horizontal supporting plane” | The trailer as a complete object, uncoupled |
| GTM — Gross Trailer Mass | “the mass transmitted to the ground by the ‘Axle’ or ‘Axles’ of the trailer when coupled to a drawing vehicle and carrying its maximum load approximately uniformly distributed over the load bearing area” | The share of the trailer carried on its own wheels |
Two supporting definitions do the rest of the work. Axle Load is “total load transmitted to the road by all the tyres of all the wheels whose centres may be included between 2 transverse parallel vertical planes less than one metre apart”. Gross Axle Load Rating (GALR) is “the ‘Manufacturer’s’ specified maximum ‘Axle Load’ for each ‘Axle’”. GVM and GCM are both manufacturer-specified values, not calculated ones; nothing in the ADRs obliges a manufacturer to set GCM equal to GVM plus the trailer rating, and in practice most do not.
The ADRs also distinguish two unladen reference masses that get used interchangeably in marketing material and should not be. Unladen Mass is “the mass of the vehicle in running order unoccupied and unladen with all fluid reservoirs filled to nominal capacity including fuel, and with all standard equipment”. Tare Mass is the same thing “except for fuel, which shall be 10 litres only, and with all standard equipment and any options fitted”. A payload figure quoted against tare is therefore larger than the same payload quoted against unladen mass, by the mass of the fuel above 10 litres — tens of kilograms on a large tank. Check which reference a figure uses before you subtract it from anything. Our companion piece on ATM, GTM and tare covers the trailer side of the same distinction.
Gross combination mass is not GVM plus ATM
Read the GCM definition again: it is the sum of the drawing vehicle’s mass and the trailer’s axle loads. The trailer’s coupling share is not added separately, because it is already inside the vehicle’s mass. Adding GVM to ATM double-counts the towball load and produces a combination figure that no rating in the ADRs corresponds to.
Two consequences follow, and both matter in practice.
First, the practical combination test is: actual laden tow-vehicle mass (which already contains the towball download) plus the actual load through the trailer’s axles must not exceed GCM. The ADRs also name the measured quantity: Total Combination Mass is “the combined mass of the laden or unladen trailer and the towing vehicle”.
Second, GCM headroom is not payload. GCM constrains the pair. GVM constrains the vehicle. A combination can sit 600 kg under GCM while the vehicle is 10 kg over GVM, because the two limits measure different things. The worked example below is built to show exactly that.
Rated towing capacity is derived, and the towbar’s number is not it
The ADR Definitions instrument defines Rated Towing Capacity as “the lesser of either; the rating given to the towing equipment fitted to the motor vehicle or, the difference between ‘Gross Combination Mass’ and ‘Gross Vehicle Mass’”.
That is a two-part test, and the second part is the one that surprises people. Whatever the towbar is stamped with, the vehicle’s rated towing capacity cannot exceed GCM minus GVM. A vehicle with a 6,000 kg GCM and a 3,100 kg GVM has a 2,900 kg ceiling on its rated towing capacity, even if a 3,500 kg towbar is bolted to it.
ADR 62/02 reinforces this from the equipment side. For towbars rated over 3.5 tonnes ATM, clause 13.5.1.3 requires the marked maximum rated capacity to be a figure “which must not exceed the vehicle manufacturer’s rated capacity”. For vehicles with a maximum rated towing capacity up to 3.5 tonnes ATM, clause 13.1 instead requires the towbar, towing bracket, attachment and vehicle structure to comply with AS 4177.1-2004 Towbars and towing brackets, a standard this article has not read and does not summarise. Either way, the towbar marking is a ceiling on the equipment, not a licence for the vehicle.
Couplings are marked separately. Under clause 12.4.1.2, a coupling for trailers up to 3.5 tonnes ATM must carry the coupling manufacturer’s name, “the maximum allowable trailer ‘ATM’” at which it is rated, and the words “use with model (identified model)”. So the coupling is rated in ATM, the towbar in rated capacity, and the vehicle in GCM minus GVM. Three numbers, three sources, all of which have to clear the same trailer.
One interpretive note, flagged as our reading rather than as a finding: because the trailer contributes its axle loads to GCM, the quantity GCM minus GVM is a GTM-side allowance, while towing equipment is rated in ATM. The ADR definition compares the two directly. Where the difference is material, take the more conservative of the two figures and ask the vehicle manufacturer in writing which basis its published towing capacity uses.
Where the towball number comes from
Because ATM includes the mass imposed on the drawing vehicle and GTM excludes it, the coupling share at the trailer’s rated maximum is simply ATM minus GTM. That is not our invention: ADR 62/02 uses the same difference explicitly. In clause 12.3.3.2.1, setting the dynamic test regime for couplings over 3.5 tonnes ATM, the standard states that “The static vertical ‘Coupling’ load (kN) = 9.81 x S” where “S = ‘ATM’ - ‘GTM’ (tonnes)”.
For a trailer plated at 2,500 kg ATM and 2,290 kg GTM, the rated coupling share is 210 kg. That 210 kg lands on the tow vehicle and is counted against its GVM, its rear axle GALR and its towbar download limit — a point covered at length in our article on towball download and vehicle payload.
A worked example, with clearly labelled hypothetical figures
Neither the vehicle nor the trailer below exists. Every figure is illustrative and chosen to make the arithmetic legible.
Illustrative tow vehicle (category NA, light goods vehicle):
| Figure | Value |
|---|---|
| Unladen Mass as built | 2,350 kg |
| GVM | 3,100 kg |
| GCM | 6,000 kg |
| Towbar marked rated capacity | 3,500 kg |
| Manufacturer’s maximum towball download | 300 kg |
Illustrative camper trailer (category TB, light trailer):
| Figure | Value |
|---|---|
| ATM | 2,500 kg |
| GTM | 2,290 kg |
| Tare | 1,760 kg |
Step 1 — check the pairing before loading anything
- Rated towing capacity = lesser of (3,500 kg towbar rating; 6,000 − 3,100 = 2,900 kg) = 2,900 kg.
- Trailer ATM 2,500 kg ≤ 2,900 kg. Passes, with 400 kg to spare.
- Coupling share at ATM = 2,500 − 2,290 = 210 kg ≤ 300 kg download limit. Passes.
- The coupling itself must be marked for at least 2,500 kg ATM under ADR 62/02 clause 12.4.1.2. Verify on the hardware.
At this point every headline number looks comfortable. Nothing has been loaded yet.
Step 2 — load the tow vehicle
Vehicle payload allowance = GVM − Unladen Mass = 3,100 − 2,350 = 750 kg.
| Added to the vehicle (illustrative) | kg |
|---|---|
| Bull bar, driving lights, winch | 90 |
| Rear drawers and cargo barrier | 65 |
| Roof rack and rooftop load | 55 |
| Long-range tank, fuel above standard capacity | 60 |
| Two occupants | 160 |
| Recovery gear, tools, fridge, camp chairs | 120 |
| Towball download from the trailer at ATM | 210 |
| Total added | 760 |
Laden vehicle mass = 2,350 + 760 = 3,110 kg against a GVM of 3,100 kg. Over by 10 kg.
Step 3 — check the combination
Laden vehicle 3,110 kg + trailer axle load 2,290 kg = 5,400 kg against a GCM of 6,000 kg. 600 kg under.
The result
| Limit | Value | Actual | Margin |
|---|---|---|---|
| Trailer ATM | 2,500 kg | 2,500 kg | 0 kg |
| Trailer GTM | 2,290 kg | 2,290 kg | 0 kg |
| Rated towing capacity | 2,900 kg | 2,500 kg | 400 kg |
| Towball download | 300 kg | 210 kg | 90 kg |
| Tow-vehicle GVM | 3,100 kg | 3,110 kg | −10 kg |
| GCM | 6,000 kg | 5,400 kg | 600 kg |
GVM binds first, by 10 kg, while GCM shows 600 kg of headroom and the towing capacity check passes with 400 kg in hand. Four accessories and two occupants did it, before a single item of camp gear entered the trailer.
Why GVM usually binds, and what actually relieves it
The reason is structural, not a matter of packing discipline. The tow vehicle carries three loads that the trailer does not offset: its own accessories, its occupants, and the coupling share of the trailer. Only the third of these shrinks when you shift weight into the trailer, and it shrinks by less than the weight you move, because a portion of anything added to the trailer returns to the vehicle through the coupling.
Three points follow for anyone trying to get back under GVM:
- Removing mass from the vehicle is the only change that helps kilogram for kilogram. Accessories are the usual candidate because they are permanent and are already on the vehicle before any trip is planned.
- Moving load from the vehicle into the trailer helps by less than the mass moved, and only while the trailer stays inside both ATM and GTM. Where that load sits in the trailer changes the coupling share, and changing the coupling share affects towing stability, which is outside the scope of this article.
- Weight-distributing hitches redistribute axle loads. They do not change any total. The laden vehicle mass, the trailer axle load, and the combination mass are unchanged by a hitch; the individual axle loads are not. See our payload explainer for the measurement sequence.
Where the four figures actually come from
For the trailer, the ADRs are unambiguous. ADR 61/03 clause 7.3.1 requires every category TA, TB, TC or TD vehicle to carry a permanently affixed metal Vehicle Plate showing at least the manufacturer’s name or vehicle make, the VIN, “the ‘Aggregate Trailer Mass (ATM)’ in kg” and “the ‘Gross Trailer Mass (GTM)’ in kg”. Two of the four masses are therefore stamped on the trailer by law. Our article on the trailer compliance plate covers what else that plate should show.
For the tow vehicle, the position is different, and it is worth stating precisely because it is widely misunderstood. Neither ADR 61/03 nor ADR 61/02 requires a GVM or a GCM figure to be marked on a category MC off-road passenger vehicle or a category NA light goods vehicle — that is, on the 4WD wagons and dual-cab utes most often used to tow campers. Under ADR 61/03, the Vehicle Plate provisions of clause 7 apply only to heavy omnibuses (clause 7.1) and to trailers (clause 7.3); light vehicles need only the Secure Vehicle Identification Marking of clause 6, which carries the manufacturer or make and the VIN. Under the earlier ADR 61/02, clause 8.1 required a Vehicle Plate only “On every ME category, NC category and T-Group vehicle”, and its GCM requirement at clause 8.1.2.5 applied to NC category vehicles — heavy goods vehicles over 12 tonnes GVM.
Where a light vehicle does carry GVM or GCM on a plate, that marking is additional information provided at the manufacturer’s discretion, or required by an instrument other than these two ADRs. In practice this means the four figures come from four places:
| Figure | Where to get it |
|---|---|
| Trailer ATM and GTM | The trailer’s Vehicle Plate, required by ADR 61/03 clause 7.3.1 |
| Tow-vehicle GVM and GCM | The vehicle manufacturer’s own specification or owner’s handbook, and the vehicle’s entry on the Register of Approved Vehicles |
| Towbar rated capacity and download limit | The towbar marking, and the vehicle manufacturer’s towing documentation |
| Coupling rated ATM | The coupling marking required by ADR 62/02 clause 12.4.1.2 |
If a seller quotes GVM or GCM from memory, ask for the manufacturer’s document. Nothing on the vehicle is legally obliged to confirm it.
What this arithmetic does not settle
- It is not a compliance certificate. The ADRs set ratings at the point of manufacture. Whether a loaded combination is lawful on the road is a matter of state and territory law, which this article does not cover and which differs between jurisdictions.
- It ignores axle loads and tyres, which can bind before GVM. A rear axle can reach its GALR while the vehicle total is still under GVM, particularly with a heavy towball download and a loaded tray. Tyre ratings are a separate constraint again; see trailer tyre load ratings.
- It does not address GVM or GCM upgrades. Those are modifications with their own approval pathways, and we have not assessed them.
- It does not read AS 4177.1-2004 or AS 2213.1-2001. Both are referenced by ADR 62/02 and neither is quoted here.
- The figures in the worked example are hypothetical. They were chosen to make one arithmetic point. Substitute your own plated and documented numbers before drawing any conclusion about your own vehicle.
- Compilations change. The definitions above are Compilation No. 16 of the ADR Definitions instrument, ADR 62/02 as compiled from 5 February 2010, ADR 61/03 as registered on 30 September 2020 and ADR 61/02 Compilation No. 3.
Disclosure and how we verified this explainer
Camper News AU is edited by Gauthier T., who works with brands in the Australian camper trailer industry, including Breath Trailer. No Breath Trailer product, figure or document was used in this article, and Breath Trailer’s trailers are subject to the ADR 61/03 plating requirement described above in exactly the same way as every other manufacturer’s. Payload carries 25 per cent in our published methodology, calculated from ATM less tare on the manufacturer’s technical data sheet; this explainer scores no one.
We downloaded the four Australian Design Rule instruments cited above from the Federal Register of Legislation on 31 August 2026 and read the authorised PDF text of each. Every quotation is taken from those texts, with the clause number given. The worked example is our own arithmetic applied to invented figures, and is labelled as such. If a later compilation changes a definition relied on here, we will update this article and log the change on the corrections page.