Towing weight ratings look like alphabet soup: GVM, ATM, GCM, GTM, TBM, Tare, Payload, Ball Weight. Most caravan owners know the terms exist but could not confidently explain what each one means, where to find it on their rig, or why exceeding any one of them makes their setup both unsafe and illegal. This guide explains every rating in plain English.
The Ratings That Apply to Your Tow Vehicle
Tare Weight is the weight of your vehicle as it rolled off the factory floor — no fuel, no passengers, no accessories. This is rarely useful in practice because nobody drives an empty vehicle, but it is the baseline from which payload is calculated.
Kerb Weight is the tare weight plus a full tank of fuel and all factory-standard fluids. This is the number you will find on your vehicle’s compliance plate and in the owner’s manual. It is the realistic starting weight of your vehicle before you add anything.
GVM (Gross Vehicle Mass) is the maximum your tow vehicle is legally allowed to weigh, fully loaded. This includes the kerb weight plus every passenger, every piece of gear in the boot, every accessory bolted on (bull bar, roof rack, drawers), and the ball weight from whatever you are towing. Exceed your GVM and you are driving an unroadworthy, uninsured vehicle.
Payload is the difference between GVM and kerb weight. This is how much you can add to your vehicle before it exceeds its legal limit. A vehicle with a GVM of 3,200 kg and a kerb weight of 2,400 kg has a payload of 800 kg. That 800 kg has to cover passengers, gear, accessories, fuel above factory spec, and the ball weight of your caravan. It goes faster than most people expect.
The Ratings That Apply to Your Caravan
Tare Weight for a caravan is the weight as delivered by the manufacturer — no water, no gas, no personal gear. This is the factory weight listed on the compliance plate. Be aware that many manufacturers weigh the van before fitting options that were ordered, so the actual tare can be 100–300 kg higher than the published figure.
ATM (Aggregate Trailer Mass) is the maximum your caravan is legally allowed to weigh when fully loaded and hitched to the tow vehicle. This is the caravan equivalent of GVM. It includes the tare weight plus everything you put in it: water, gas, food, clothes, tools, spare tyre, and the kitchen sink you swore you would not bring.
Payload for the caravan is ATM minus Tare. A caravan with an ATM of 3,500 kg and a tare of 2,700 kg has a payload of 800 kg. Sounds generous until you add 200 litres of water (200 kg), two 9 kg gas bottles (36 kg), and a few hundred kilos of gear. Many vans are overweight before the owners finish packing.
GTM (Gross Trailer Mass) is the maximum weight allowed on the caravan’s axle group when connected to the tow vehicle. It is always less than or equal to the ATM, because some of the caravan’s weight transfers forward through the coupling as ball weight. GTM matters for axle and suspension ratings.
TBM (Tow Ball Mass) or Ball Weight is the downward force the caravan exerts on the tow vehicle’s hitch. This weight transfers directly to your vehicle and counts toward its GVM. Typical ball weight is 8–12% of the loaded caravan weight. Too little ball weight causes sway. Too much overloads the rear axle and lifts the front of the tow vehicle, reducing steering and braking.
The Rating That Connects Both
GCM (Gross Combination Mass) is the maximum total weight of the tow vehicle and caravan combined when both are fully loaded. This is specified by the tow vehicle manufacturer and is listed on the vehicle’s compliance plate or in the owner’s manual.
Here is the critical subtlety: GCM is not simply GVM plus ATM. In many vehicles, the GCM is less than GVM + maximum towing capacity. This means you cannot legally max out both. If your vehicle has a GVM of 3,200 kg and a maximum towing capacity of 3,500 kg, but a GCM of 6,000 kg, then loading the vehicle to its 3,200 kg GVM leaves only 2,800 kg for the caravan — not the full 3,500 kg towing capacity.
This catches more people than any other weight rating. You can be under GVM and under ATM individually, but still over GCM in combination.
Where to Find Your Ratings
Your tow vehicle’s GVM, GCM, and towing capacity are on the compliance plate (usually on the driver’s door jamb or under the bonnet) and in the owner’s manual. Your caravan’s Tare, ATM, and GTM are on its compliance plate, typically on the drawbar or A-frame near the coupling.
If your vehicle or caravan has been modified — GVM upgrade, chassis reinforcement, suspension changes — the modified ratings should be on a secondary compliance plate from the engineering company that did the work. Use the modified ratings, not the original factory figures.
What Happens If You Exceed a Rating
Exceeding any weight rating makes your vehicle unroadworthy. In every Australian state and territory, driving an overloaded vehicle is an offence that carries fines from $400 to over $5,000 depending on the jurisdiction and severity. If you are involved in an accident while overloaded, your insurance company has grounds to deny your claim entirely — for both vehicle damage and third-party liability.
Beyond the legal consequences, the physics are unforgiving. Overloaded tyres run hotter and fail sooner. Overloaded brakes take longer to stop and fade faster on descents. An overloaded rear axle lifts weight off the front wheels, reducing steering response and increasing stopping distance. None of these failure modes announce themselves gradually. They tend to happen suddenly, at highway speed, on a hot day.
How to Check Your Numbers
The only reliable way to know your actual weights is to weigh your rig. Load your tow vehicle and caravan the way you travel, drive to a public weighbridge, and weigh each unit individually and in combination. Compare every measured weight against the corresponding rating. If any single figure exceeds its limit, you need to remove weight or upgrade the rating.
Ready2Tow builds your complete weight picture digitally. Enter your vehicle specs and the app checks GVM, ATM, GCM, GTM, ball weight, and per-axle loads against the legal limits. Colour-coded results show exactly which ratings are safe and which need attention. The comparison page shows what Ready2Tow does that spreadsheets and guesswork do not.
