Reviewed August 2026 · General information
Safe matching requires every limit to work at the same time. ATM, GTM, GVM, GCM, axle limits and tow-ball download describe different parts of the combination; staying below one number does not guarantee the rest are compliant.
The terms that matter
- Tare mass
- The manufacturer's specified mass of the unladen trailer in its defined condition. Confirm what was and was not included when the figure was established.
- ATM — Aggregate Trailer Mass
- The maximum permitted total mass of the loaded trailer when it is not coupled to a tow vehicle, including the mass carried through the coupling.
- GTM — Gross Trailer Mass
- The maximum permitted mass carried by the trailer's wheels when the loaded trailer is coupled to the tow vehicle.
- Payload
- The capacity available for options, water, gas, food and gear. A plated calculation may use ATM minus tare, but actual remaining capacity depends on the camper's real as-delivered and loaded mass.
- Tow-ball mass
- The downward load imposed on the tow vehicle through the coupling. It consumes tow-vehicle payload and can materially affect rear-axle load.
- GVM — Gross Vehicle Mass
- The maximum permitted loaded mass of the tow vehicle, including people, fuel, accessories, cargo and coupling load.
- GCM — Gross Combination Mass
- The maximum permitted combined mass of the loaded tow vehicle and loaded trailer, as specified for the vehicle.
- Axle limits
- The permitted loads on individual vehicle and trailer axles or axle groups. These can be exceeded even when GVM or ATM is not.
How the numbers interact
- Actual trailer mass is approximately the trailer axle-group reading plus the actual coupling load.
- The tow-ball load becomes part of the tow vehicle's loaded mass while coupled.
- Passengers, accessories, fuel and cargo reduce the vehicle payload available for tow-ball load.
- The actual combined mass must remain below GCM, while the vehicle, trailer, coupling and every axle remain within their individual limits.
- A rule-of-thumb ball-load percentage is not a substitute for measured loads, the manufacturer's limits and stable loading.
A practical weighbridge sequence
Ask the weighbridge operator what their platform can measure and take your vehicle and camper in normal travel condition: people, fuel, water, gas, food, tools and usual equipment.
| Measurement | What it helps establish |
|---|---|
| Tow vehicle and trailer together | Actual combined mass for comparison with GCM |
| Tow vehicle axles while coupled | Loaded vehicle mass and front/rear axle distribution |
| Trailer axle group while coupled | Actual mass carried through the trailer wheels |
| Trailer uncoupled, where the site permits | Actual total trailer mass |
| Coupling load using suitable equipment | Actual tow-ball download and its effect on the vehicle |
A simple pre-trip check
- Confirm the exact vehicle variant's towing, GVM, GCM, axle and tow-ball limits.
- Use the camper compliance plate and current approved specifications.
- Account for dealer and owner-fitted accessories on both vehicles.
- Measure again after major equipment, battery, water or storage changes.
- Distribute and secure load according to manufacturer guidance; do not move heavy items merely to manipulate one reading.
Where to get a definitive answer
If any limit or calculation is unclear, use an accredited weighbridge and seek advice from the vehicle and trailer manufacturers or a suitably qualified automotive engineer. State rules and licence conditions also apply.
Authoritative sources
Specifications and regulations can change. Check the latest applicable material before making a towing or safety decision.
Want advice around your own setup?
Bring your tow-vehicle details, intended trips and likely load so the conversation can start with the complete combination.
Talk to the team