If you've ever stared at the compliance plate on a caravan and walked away more confused than when you started, you're not alone. Australian towing law uses five different weights to describe what your rig can carry — and exceeding any one of them is enough to land you in trouble. The good news: once you understand what each one actually measures, it stops feeling like alphabet soup.
Here are the five weights you need to know, in plain English.
1. GVM — Gross Vehicle Mass
What it measures: the maximum total weight of your tow vehicle when fully loaded — that includes the vehicle itself, driver, passengers, fuel, all your gear in the back, AND the downward force the caravan puts on the tow ball.
Where you'll find it: on the vehicle's compliance plate, usually in the door jamb.
Why it's the one most people exceed: dual-cab 4WDs are heavier than people realise once you add a canopy, drawers, fridge, spare tyres, recovery gear, water, and a family. Hitching up a heavy van eats into the rest. Many setups blow GVM before the caravan is even fully loaded.
2. ATM — Aggregate Trailer Mass
What it measures: the maximum total weight of your caravan or trailer when it's not coupled to a tow vehicle — everything in and on the van, plus the van itself, sitting on its own wheels and jockey wheel.
Where you'll find it: on the caravan's compliance plate.
Why it matters: ATM is what your van rates to. Exceed it and the caravan itself is technically non-compliant — regardless of what tow vehicle you're using.
3. GTM — Gross Trailer Mass
What it measures: the weight of your caravan on its own wheels when it's coupled to the tow vehicle. In other words, the weight transferred to the ground through the trailer's axles — not including the weight resting on the tow ball.
The simple relationship: GTM = ATM − tow ball weight. Couple the van up, weight shifts forward onto the tow ball, and the weight on the trailer's wheels drops. That weight on the wheels is GTM.
Why it matters: GTM is rated separately on the compliance plate and is what the trailer's brakes and suspension are designed around. You can be within ATM but over GTM if your van is loaded heavy at the back.
4. Tow ball weight (sometimes called tow ball download)
What it measures: the downward force the caravan applies to the tow vehicle through the coupling.
Two ratings to check: your tow vehicle has a maximum rated tow ball weight, and the caravan does too. You're limited by the lower of the two.
The trap nobody warns you about: tow ball weight isn't directly listed on the van's compliance plate — you can only measure it. Most rigs target 8–15% of ATM as a healthy range. Too low and the van will sway. Too high and you're overloading both ends of the coupling.
5. GCM — Gross Combination Mass
What it measures: the maximum combined weight of your tow vehicle plus your loaded caravan. It's the rating that says "this is the total your tow vehicle can drag down the road."
Where you'll find it: stamped on the tow vehicle's compliance plate, often the easiest to miss.
Why it matters: you can be under GVM and under ATM individually and still be over GCM if the two together exceed what the tow vehicle is rated to combine. This is where a lot of upgrade decisions get made — GCM is usually the binding limit.
A worked example
Say your 4WD has a GVM of 3,300 kg and a GCM of 6,000 kg. Your van has an ATM of 2,800 kg. Even if you load the van right to its 2,800 kg ATM and keep the tow vehicle under 3,300 kg, the combined total can't exceed 6,000 kg. That leaves you 3,200 kg of tow vehicle to play with — not the 3,300 kg the GVM suggests. The lower of the two limits always wins.
So which one will catch you out?
In our experience, the most commonly exceeded limits, in order, are:
- Tow ball weight — because nobody measures it without a scale.
- GVM of the tow vehicle — canopies, drawers, fridges, water and family creep up on you.
- GCM — the silent killer; everything else looks fine until you do the maths.
- ATM — less common, but easy to do on long trips with lots of water and gear.
- GTM — usually only an issue on heavily back-loaded vans.
The trick with all of this: the numbers on the compliance plates are measured unladen. The numbers that actually matter to police, insurers and you — are the loaded ones. Those you have to measure.
That's the entire reason mobile weighing exists. See exactly what we measure on-site, or read up on where we operate across the Albury Wodonga region and Southern Riverina NSW.