Towing Weights Explained: ATM, GVM, GCM, Tow Ball and Payload

More rigs on the road are overweight than the owners would ever believe, and most found out the hard way: at a weighbridge, after an insurer wriggled out of a claim, or when the back of the car sagged like a hammock the day they loaded up for the big trip. The letters ATM, GVM and GCM aren't red tape. They're the numbers that decide whether your rig is safe, legal and insured, and they're not hard once someone lays them out plainly. So here they are.

The five numbers that matter

• Tare is the empty weight of the van as it left the factory, no water, no gear, sometimes no gas or battery. It's the number the salesroom quotes and the least useful for planning.
• ATM (Aggregate Trailer Mass) is the most your loaded van is allowed to weigh, measured with the van standing on its own wheels and jockey wheel, so it includes everything you've packed plus the tow ball download. This is your van's legal ceiling.
• GTM (Gross Trailer Mass) is the weight carried by the van's own axles only, measured while it's hitched to the car. It's the ATM minus whatever is pressing down on the tow ball. Bearings, axles and tyres are rated to GTM.
• Tow ball mass is the download your loaded van puts on the car's tow ball, often around 8 to 10 per cent of the van's weight for stable towing. This weight lands on the car and counts against the car, which trips up a lot of people.
• GVM (Gross Vehicle Mass) is the most your loaded tow car is allowed to weigh, including passengers, fuel, gear and that tow ball download sitting on its back axle.
• GCM (Gross Combination Mass) is the most the car and van are allowed to weigh together, fully loaded, at the same time. This is the one that quietly sinks the most rigs.

Why the tow ball is the sneaky one

The tow ball mass is the trap, because it counts twice in your thinking. It's part of the van's ATM, and the moment you hitch up it transfers onto the car and eats into the car's GVM and its rear axle rating. So a van that's legal sitting in the driveway can shove your fully-loaded car over its GVM the instant you connect it. People load the car to the brim, load the van to the brim, hitch up, and now the car is illegally overweight without a single extra thing being added. Nothing warns you. The suspension just squats.

There's a second trap in the other direction that's worth a mention, because it's about safety rather than legality. Too little tow ball weight is dangerous too. A van loaded tail-heavy, with the weight behind the axle, can drop the tow ball mass too low and start to sway at highway speed, and trailer sway is genuinely frightening and can end very badly. That's why the rough 8 to 10 per cent figure exists: enough download to keep the van tracking straight behind you, without so much that it buries the back of the car. Where you put your heavy gear in the van changes this number, so pack the heavy stuff low and over the axle, not in a box at the very back.

A worked example

Say your tow car has a GVM of 3200kg and a GCM of 6000kg, and it's rated to tow 3000kg. Your van has an ATM of 3000kg. Looks tidy: 3000 towing capacity, 3000 ATM. Watch what happens.

Now the car alone is at 3350kg against a 3200kg GVM. Overweight before you've left the driveway. And the combination is 3050 + 3000 = 6050kg against a 6000kg GCM, so you're over on GCM too. Both numbers looked fine in the brochure. Loaded and hitched, the rig is illegal on two counts. This is not a rare edge case. It's the standard way a "matched" car and van turn out over the line, and it's why I tell everyone to weigh the real rig, loaded, at a public weighbridge rather than trust the plates and the salesman's arithmetic.

Why so many rigs are over

Three reasons, over and over. Vans have crept heavier while payloads shrank, so a modern off-road van loaded with a big battery bank, full water and two spares can eat its whole payload before you pack a plate. Batteries are a chunk of it: a big lithium and inverter setup is real weight, which is one more reason to size it honestly, and the comparison in lithium vs AGM spells out the kilograms. Second, people forget the tow ball counts against the car. Third, water is heavy and invisible in the sums: 100 litres is 100kg, and a fully-plumbed off-road tourer carries a lot of it. A fourth, quieter reason is that people plan for the trip they're on and forget the trip they're building toward: bikes on the rear rack, a bigger fridge, a roof of solar, a full toolbox and a pantry stocked for a month all creep in over the first season, and each addition eats payload you thought you had. Weigh again after you've truly finished loading for a long haul, not just after the shakedown weekend, because the rig that was legal in March can be over by June without a single deliberate decision.

What a weighbridge visit actually looks like

People imagine weighing a rig is a hassle. It's an hour and a few dollars, and it's the cheapest insurance you'll ever buy. A public weighbridge (many are at recycling depots, quarries and transport yards) will weigh you in stages if you ask. The sequence I use is simple. Drive the whole rig on and get the combined weight, which is your GCM check. Then unhitch and drive the car off, leaving the van sitting on its own wheels, and weigh the van alone, which is close to your ATM. The difference between the combined figure and the van-alone figure, compared against the car alone, tells you roughly what's landing on the tow ball.

Better still, if you can weigh one axle group at a time, do it, because the number that catches people isn't always the total. A rig can be under its overall GVM and still be over on the rear axle, which is exactly what a heavy tow ball download does. Axle overloads are their own offence and they're the ones that chew tyres and bearings. Do this loaded the way you actually travel, with full water, full fridge, the kids and the dog and the annexe, not empty on the way home from the dealer. An empty weigh tells you almost nothing useful.

How to get legal and stay there

Weigh it loaded, the way you actually travel, at a weighbridge, and get each number: the car alone, the van's GTM, the tow ball, and the combined figure. If you're over GVM on the car, a GVM upgrade is a legitimate engineering path in Australia and worth pricing. If you're over on payload in the van, the honest fixes are less water for the leg, fewer spares, and getting ruthless about what you carry. Getting the weights right is step one of choosing the rig in the first place, which is why our guide to choosing an off-road caravan starts with payload, and it feeds straight into how heavy you can safely run on rough surfaces, covered in tyre pressures for corrugations. Match all of it to where you're headed with our rig-rating system. The heavier authority on the exact definitions is the National Heavy Vehicle Regulator and your state road authority; when in doubt, weigh it, because a weighbridge ticket never lies and a brochure often does.