Plenty of 4WDs towing caravans in this country are closer to their legal limits than their owners realise, and a fair number are over. That is rarely recklessness. The weights interact in ways the brochure never explains, and four numbers govern the whole thing. Once you can name them, the maths takes ten minutes and a trip to a weighbridge.
- GVM (Gross Vehicle Mass): the most your loaded vehicle can weigh on its own.
- Payload: GVM minus the vehicle's kerb weight. Everything you add lives in here.
- GCM (Gross Combination Mass): the most the vehicle and trailer can weigh together.
- ATM and GTM: the trailer's total mass, and the part of it carried on its own axles once hitched.
Why payload disappears faster than you expect
A dual cab with a 1,000kg payload sounds generous until you load it up and add it all up:
- Bull bar, winch and underbody protection: 90kg to 150kg
- Canopy or hard lid: 60kg to 120kg
- Drawers, fridge and recovery gear: 120kg to 180kg
- Water, 80 litres: 80kg
- Long-range fuel, full: 60kg to 120kg over standard
- Four adults: around 320kg
- Tow ball download on a 2,800kg van at 10 per cent: 280kg
That is over a tonne before the esky goes in. The last line is the one that catches people. Tow ball download is not carried by the trailer. It sits on your vehicle and comes straight out of your payload, which is how a heavy van can push an otherwise legal 4WD over its GVM the moment it is hitched.
The GCM trap
This part surprises even experienced tourers. Your vehicle might be rated to tow 3,500kg and have a GVM of around 3,300kg, yet its GCM is often well short of those two numbers added together. When that is the case, towing at the maximum means you cannot load the vehicle to its own GVM. The towing capacity and the vehicle capacity cannot both be used in full at the same time.
Treat it as a subtraction, not an addition. Start with GCM, take away the actual weight of your loaded trailer, and what is left is the most your vehicle is allowed to weigh, fully loaded, people included. If that figure is below your GVM, then GVM is no longer your limit. GCM is.
Where a GVM upgrade helps, and where it does not
A certified GVM upgrade raises the legal maximum weight of the vehicle itself, and your usable payload with it. Depending on the vehicle and the kit, an upgrade can add anywhere from about 300kg to 1,000kg of legal carrying capacity. For a touring 4WD carrying a serious fitout, it is often the difference between a legal vehicle and an illegal one, and it comes with the suspension to carry the weight properly instead of sagging under it.
Upgrades come in two forms. A pre-registration upgrade is completed before the vehicle is first registered and certified for that vehicle, which suits new builds planned around their final load. A post-registration upgrade is done on a vehicle already on the road. It may need engineering certification, and the registration requirements depend on your state. Our GVM upgrade range covers both, with the engineering and compliance handled from the Brookvale workshop.
What a GVM upgrade does not automatically do is raise your GCM. Some upgrades include a GCM increase and some do not, depending on the vehicle, the kit and the certification path. If your reason for upgrading is towing rather than carrying, ask that question first, before you book. Our guides on what a GVM upgrade is and why you might need one and why upgrades are becoming a necessity cover the process. This page is about working out which limit actually binds on your setup.
Weigh it, do not estimate it
Every number above is theory until the vehicle goes over a weighbridge. Weigh it loaded for a real trip, with water, fuel, people and the van hitched, and get the individual axle weights as well as the totals. An axle limit can be exceeded while the overall weight looks fine.
Then fix what the scales tell you. Sometimes that means moving weight forward of the rear axle and getting the heavy items off the roof. Sometimes it means leaving gear at home. And sometimes it means a suspension or GVM upgrade, which is the right answer for a vehicle that is carrying more than it was built to.
Frequently asked questions
What is the difference between GVM and GCM?
GVM is the maximum weight of the loaded vehicle on its own. GCM is the maximum weight of the vehicle and trailer combined, and it is often less than GVM plus towing capacity.
Does tow ball weight count as payload?
Yes. Tow ball download sits on the vehicle and reduces your available payload, typically by around 10 per cent of the trailer's loaded weight.
Can I tow 3,500kg and still load my vehicle to GVM?
Often not. Check your GCM: subtract the loaded trailer weight from it, and the remainder is all your vehicle can legally weigh.
Does a GVM upgrade increase towing capacity?
It increases the vehicle's own legal weight and payload. Whether GCM rises as well depends on the vehicle, the kit and the certification, so confirm it before you book.
Want to know where your setup actually sits? Call the Brookvale workshop on 1300 856 888 or book a GVM consultation, and bring your weighbridge figures so we can work through the numbers together.
