Short answer: GVM (gross vehicle mass) is the most your 4WD or ute is allowed to weigh fully loaded. GCM (gross combination mass) is the most the vehicle and whatever it is towing can weigh together. ATM (aggregate trailer mass) is the most a trailer or caravan can weigh fully loaded, including the weight pressing down on your tow ball. GTM (gross trailer mass) is the loaded trailer's weight on its own tyres, without that ball load. Tow ball mass (TBM, or ball weight) is the weight the trailer puts on your vehicle, and it counts against your vehicle's payload and GVM.
These numbers come up every week in our Gawler South workshop, usually after someone has added a bull bar, canopy, drawers and a fridge, then hooked up a caravan or camper trailer. Here is how each rating works, where to find it, and how they stack up once you tow.
The vehicle ratings
Tare and kerb mass
Tare or kerb mass is roughly the weight of the empty vehicle. Manufacturers define it slightly differently, so treat the brochure figure as a starting point only. What matters is what your vehicle weighs as you actually drive it, with people, fuel, water and gear on board.
GVM: gross vehicle mass
GVM is the maximum loaded mass of the vehicle, set by the manufacturer. You will find it on the vehicle's plate and in the owner's manual. Everything counts towards it: passengers, fuel, water, accessories bolted on after delivery, everything in the canopy or the back seat, and the weight sitting on the tow ball.
Payload
Payload is the gap between what your vehicle weighs empty and its GVM. Accessories eat into it before you pack a single bag. As our bull bars and protection page notes, a steel bull bar is typically 70 to 95 kg on its own. Add a canopy, drawers, a second battery, a long range tank full of diesel and a couple of passengers, and a loaded dual cab can run short of payload quickly.
The trailer ratings
ATM: aggregate trailer mass
Vehicle Standards Bulletin 1 (VSB 1), the federal guide for trailers up to 4.5 tonnes, defines ATM as the total mass of the trailer when carrying the maximum load recommended by the manufacturer, including the mass imposed on the towing vehicle. It is on the trailer's plate. ATM minus the trailer's tare mass is how much you can load into it. VSB 1's own example: a car trailer with a 700 kg tare and a 2,000 kg ATM can carry a car of up to 1,300 kg.
GTM: gross trailer mass
GTM is the maximum mass the fully loaded trailer puts through its own tyres while it is coupled to the tow vehicle. It does not include the load on the tow ball. Trailer brake requirements are based on GTM, not ATM.
Tow ball mass (TBM)
VSB 1 puts it simply: with the trailer at maximum load and loaded evenly (or as the trailer maker specifies), ATM minus GTM equals the tow ball down-weight. That weight sits on your vehicle, usually behind the rear axle, so it uses payload and counts towards GVM. Vehicle manufacturers also set a maximum tow ball load, so check the owner's manual and the tow bar's rating and stay under the lower of the two.
The combination ratings
Towing capacity, braked and unbraked
Braked towing capacity is the heaviest braked trailer the vehicle maker says your vehicle can tow. For a trailer without brakes the limit is 750 kg, or less if your vehicle maker specifies a lower unbraked figure. In South Australia, the loaded trailer must not exceed the rated capacity of the tow bar and coupling, the towing mass specified by the vehicle manufacturer, or the GCM on the vehicle's registration, whichever is lowest. Tow bars made after 1 July 1988 have to show their maximum load on a plate or stamped on the bar, and VSB 1 requires 50 mm tow ball couplings to be marked with their rating (750 kg, 2,000 kg or 3,500 kg).
GCM: gross combination mass
GCM is the maximum loaded weight of the towing vehicle plus everything it is towing. This is the rating that catches people out. If your vehicle's GCM is less than its GVM plus its braked towing capacity, you cannot max out both at once: load the vehicle to its GVM and you may have to tow less than the advertised capacity, or tow at full capacity and carry less in the vehicle.
A worked example (illustrative numbers)
The figures below are made up to show the maths. They are not the ratings of any particular model, so always use the numbers on your own vehicle, tow bar and trailer plates.
| Rating or measurement | Example figure |
|---|---|
| Vehicle GVM | 3,200 kg |
| Vehicle GCM | 6,000 kg |
| Braked towing capacity | 3,500 kg |
| Vehicle weighed loaded for the trip (not hitched) | 3,050 kg |
| Caravan ATM | 2,900 kg |
| Caravan weighed loaded (total) | 2,800 kg, with 280 kg of that on the tow ball |
- Caravan vs ATM and towing capacity: 2,800 kg is under the 2,900 kg ATM and under the 3,500 kg towing capacity. Pass.
- Combination vs GCM: 3,050 kg + 2,800 kg = 5,850 kg, under the 6,000 kg GCM. Pass.
- Vehicle vs GVM once hitched: 3,050 kg + 280 kg of ball weight = 3,330 kg, which is 130 kg over the 3,200 kg GVM. Fail.
Everything looks fine on the caravan side, yet the vehicle is overloaded because of the ball weight. The fixes are to take weight out of the vehicle, move load in the van within the van maker's guidance, or look at a certified GVM upgrade, checking what it does to your GCM before you rely on it.
How to check your own numbers
- Collect the ratings. Vehicle plate and owner's manual for GVM, GCM, towing capacity and maximum ball load; the tow bar's plate or stamping; the trailer's plate for ATM, GTM and tare.
- Weigh, don't guess. Weigh the vehicle loaded for a trip, then the trailer, then the two hitched together at a public weighbridge. The difference tells you the ball weight and the real numbers to compare against each rating.
- Check the trailer brakes. Under VSB 1, trailers over 750 kg GTM need brakes. Over-run brakes are allowed up to 2,000 kg GTM. Over 2,000 kg GTM, the brakes must work on all wheels and a breakaway system must apply them and hold them on for at least 15 minutes if the trailer comes off.
- Set the vehicle up for the load. Our towing setups cover tow bars, wiring, electric brake controllers, weight distribution hitches, sway control and towing mirrors, and suspension upgrades help a loaded vehicle sit and handle properly. Neither raises your legal limits on its own.
Where a GVM upgrade fits
If the weighbridge says you are over GVM and you cannot shed the weight, a certified GVM upgrade raises the legal limit for the vehicle. Every GVM job we do starts with a full vehicle assessment and weighing, and includes heavy-duty suspension, brake upgrades where required, engineering compliance certification and the registration update with SA Transport. Read pre-rego vs post-rego GVM upgrades in SA for how the two routes differ.
Frequently asked questions
Does tow ball weight count towards my GVM?
Yes. The tow ball mass is carried by your vehicle, so it uses up payload and counts towards GVM just like a passenger or a full fridge. A 250 kg ball load on a vehicle with 300 kg of payload left leaves you only 50 kg for everything else.
Can I always tow my vehicle's full braked towing capacity?
Only if every other limit still works at the same time. The trailer can't exceed the tow bar or coupling rating, the vehicle maker's towing mass or the GCM, whichever is lowest, and the loaded vehicle (including the ball weight) must stay under its GVM.
What is the difference between ATM and GTM?
ATM is the total mass of the fully loaded trailer, including the weight it puts on the tow ball. GTM is the mass the fully loaded trailer puts through its own tyres while hitched up. The difference between the two is the tow ball down-weight.
Do trailers over 750 kg need brakes?
Under VSB 1, trailers over 750 kg GTM need an efficient service brake. Over-run brakes are allowed up to 2,000 kg GTM, and above 2,000 kg the brakes must work on all wheels with a breakaway system that holds them on for at least 15 minutes.
Will a GVM upgrade let me tow more?
Not automatically. A GVM upgrade raises how much the vehicle itself can weigh. Whether your towing capacity or GCM changes depends on the kit and its certification, so check the new ratings on the paperwork before you plan around them.
Not sure if you're over weight?
Tell us your vehicle, what you've fitted and what you tow. We'll help you work out where you stand and quote any towing, suspension or GVM work before it starts.
Get a QuoteCall 0481 569 90734 Adelaide Road, Gawler South SA 5118 · Mon to Fri 9am to 5pm, Sat 10am to 2pm
Sources
- VSB 1 Revision 6 (August 2026): Trailers with an ATM of 4.5 tonnes or less, Department of Infrastructure
- Light vehicle towing and trailer regulations, sa.gov.au
- Light vehicle mass and loading requirements fact sheet, SA Police
- Light vehicle mass re-rating, sa.gov.au
General information only, current as at 5 October 2026. Ratings and rules can change; always check your own vehicle and trailer plates and the official sources above.