What is GCM (Gross Combination Mass)?

GCM is the most your tow vehicle and trailer are allowed to weigh together while hitched. Here is why it can bite before GVM or towing capacity do.

GCM in one sentence

Gross combination mass (GCM) is the maximum a tow vehicle and whatever it is towing may weigh together, hitched, as set by the tow vehicle's manufacturer. Like GVM, it is a rating: a limit you stay under, not something you read off a scale.

Some manufacturers and motoring clubs call it gross combination vehicle mass (GCVM). Not every vehicle has one published. Transport for NSW notes that where the manufacturer does specify a GCM, you must not exceed it.

Why GCM exists

A vehicle's GVM is about what the vehicle can carry. Its towing capacity is about what it can pull. GCM covers the whole combination: what the drivetrain, brakes and cooling can manage when the vehicle is both loaded and towing.

The key point is that GCM is often less than GVM plus towing capacity. The WA Department of Transport says this in its Safe Towing Guide. So a vehicle rated for 3,500 kg towing may not be able to tow 3,500 kg while it is also loaded to its GVM.

How the numbers add up

The Australian Design Rules define GCM as the maximum of the vehicle's GVM plus the axle loads of the trailer. That wording matters:

The vehicle's mass, hitched, already includes the tow ball mass. The downward load at the coupling is carried by the vehicle's rear axle.
The trailer's axle load is what the trailer puts on the ground through its own tyres while hitched. That is the trailer's GTM figure, and it excludes the tow ball mass.

So when you check a rig against its GCM, you add vehicle (hitched) + trailer axles (hitched). That is the same as everything on the weighbridge together. You do not add the full trailer mass on top of the hitched vehicle, or the tow ball gets counted twice.

A worked example

Invented figures, for illustration only:

	Rating	Actual (hitched)
Vehicle GVM	3,200 kg	3,100 kg (includes 250 kg tow ball mass)
Trailer axle load (GTM)	2,800 kg	2,650 kg
Combination	GCM 5,700 kg	5,750 kg

Each part is within its own limit: the vehicle is under GVM and the trailer is under its axle rating. The combination is still 50 kg over GCM. This is the most common way GCM is exceeded. No single figure looks wrong, and the total is still too high.

The fix is not always less gear in the trailer. Taking 50 kg out of either the vehicle or the trailer brings the combination under, though moving mass between them also changes the tow ball mass and axle loads.

GCM and towing capacity are different

Towing capacity (sometimes "maximum towing mass" or "braked towing capacity") is the most the vehicle may tow. GCM is the most the vehicle and trailer may weigh together. You need to be within both, plus the towbar's rating, the coupling's rating and the trailer's own ATM. Transport for NSW's advice is that the lowest of these limits is the one that applies to you.

The rules also differ between states on how towing capacity is read. Queensland, for example, allows a trailer whose ATM rating is above the vehicle's towing capacity, provided the trailer is not loaded beyond that capacity. Check your state road authority for the rules that apply to you.

How to check yours

Find the GCM in the owner's manual or the manufacturer's towing specifications for your exact variant.
Weigh the rig hitched and loaded as you travel, with fuel, water, passengers and gear. See how to weigh your rig.
Compare the combined reading with GCM, and each part against its own limit.

Model your own rig

LoadSavvy's Load check models your GCM together with GVM, each axle, the tow ball and the trailer's ATM and GTM. You can see which limit you reach first, and how moving a load changes it. It is a planning tool, not a certified weight. A weighbridge measures your rig.

Model your rig in the Load check →