ATM, GTM and tare are not three ways to describe the same weight. They identify different states or ratings of a trailer, and confusing them can hide an overloaded trailer, axle group or tow vehicle.
The current Australian Government Vehicle Standards Bulletin 1 (VSB1 Revision 6, August 2026) provides the national definitions used for low-ATM trailers. Owners should compare those ratings with measured, trip-ready masses and every relevant vehicle, coupling, axle and tyre limit.
The definitions in plain English
| Term | What it describes | Coupling load included? | Rating or measured value? |
|---|---|---|---|
| ATM | Maximum total mass of the loaded trailer when not coupled to a tow vehicle | Yes | Manufacturer rating |
| GTM | Maximum mass carried by the trailer tyres when coupled to the tow vehicle | No | Manufacturer rating |
| Tare | Mass of the trailer in the defined ready-for-service, unladen condition | Yes, because the trailer is uncoupled | Recorded specification |
| Actual loaded trailer mass | What the packed trailer weighs at that moment | Depends on weighing method | Measurement |
The short version is: ATM is the trailer’s total permitted mass; GTM is the permitted portion carried through its wheels while coupled; tare is an unladen reference condition. None tells you the tow vehicle’s actual axle loads.
Aggregate Trailer Mass
VSB1 defines Aggregate Trailer Mass as the maximum total mass of the loaded trailer when carrying its maximum load and when not coupled to a towing vehicle. Because the uncoupled trailer is fully supported by its tyres and coupling or jockey wheel, the coupling share is part of ATM.
ATM is a limit, not a target. Filling storage space or tanks does not authorise loading up to ATM if another lower limit is reached first. The tow vehicle’s towing capacity, GCM, GVM, axle ratings, towbar, coupling and tyres can all constrain the combination.
For an actual check, the complete loaded trailer must be measured using a method that captures the whole trailer. A wheel-only reading while coupled is not an ATM measurement because it omits the load transferred through the coupling.
Gross Trailer Mass
VSB1 defines Gross Trailer Mass as the maximum mass transmitted to the ground by the tyres of the loaded trailer when coupled to a towing vehicle. The coupling carries the remainder of the trailer’s actual total mass.
NSW Government guidance illustrates this distinction: GTM is measured under all trailer tyres while the trailer is connected to the tow vehicle. That is why GTM is normally lower than ATM.
GTM matters for braking and trailer running gear. VSB1 states braking requirements in terms of GTM, not ATM. It also appears on the mandatory vehicle plate for trailers covered by the bulletin.
Do not estimate actual coupling load merely by subtracting a random wheel reading from the plated ATM. The subtraction only becomes meaningful when measurements describe the same actual loaded condition and are taken correctly.
Tare mass
The RAV guide describes tare for a vehicle other than a motorcycle or motor tricycle as its ready-for-service, unoccupied and unladen mass, with fluid reservoirs at nominal capacity except for a specified fuel condition. VSB1 applies the relevant trailer definition and requires tare on the vehicle plate.
For camper trailers, owners should not assume that every commonly fitted item is included in the recorded tare. Accessories installed after the recorded configuration, batteries, gas, water and customer options can change the empty or travelling mass. Ask the supplier what exact configuration was used and verify the current trailer by weighing.
Tare is useful for understanding the starting point, but it is not the number that proves a packed trailer is compliant.
The new load-carrying-capacity statement
The August 2026 VSB1 update added a caravan-specific load-carrying-capacity section. It describes the minimum load carrying capacity that a newly built caravan must be designed to have, calculated from ATM minus tare, with rules tied to sleeping berths.
That manufacturing requirement should not be misread as a promise that every owner can add that much arbitrary gear. Actual accessories and fluids consume capacity, and the full combination still must remain within all other limits. A camper trailer’s classification and approval details also matter; use the current official document rather than applying a caravan paragraph without checking scope.
A worked relationship without invented numbers
Assume a trailer has plate values for ATM, GTM and tare. The relationships are:
- Rated payload reference: ATM minus tare.
- At a particular loaded condition: actual total trailer mass equals actual mass on trailer wheels plus actual coupling load.
- Remaining ATM margin: ATM minus actual total trailer mass.
- Remaining GTM margin: GTM minus actual mass on trailer wheels.
Use real plate values and measured masses in those formulas. Do not copy sample values from an article into a safety decision.
The coupling load then becomes load on the tow vehicle and can affect its GVM and rear axle. It does not disappear from the combination.
Where to verify the ratings
Check the physical vehicle plate and the trailer’s RAV record. VSB1 says the mandatory plate details include manufacturer or make, VIN, ATM, GTM and tare, and that those details must match the RAV.
If the plate is unreadable, altered or inconsistent with the RAV, do not choose the most convenient number. Ask the manufacturer and the relevant registration or engineering authority to resolve it.
The owner’s measurement checklist
- Photograph the plate and record the 17-character VIN.
- Search the public RAV and compare ATM, GTM and tare.
- List every permanent modification and accessory.
- Pack the trailer exactly as it will travel.
- Measure the complete loaded trailer with a suitable facility or competent weighing provider.
- Measure trailer-wheel load and coupling load using an appropriate method.
- Measure the loaded tow vehicle and axle loads.
- Compare every result with the lowest applicable rating.
- Repeat after a material modification or packing change.
Definitions create a common language, but compliance depends on the actual combination. Use ATM, GTM and tare for the jobs they were designed to do, then prove the travelling setup with measurements.