Towing: GCM, GVM, ball weight, and why "3.5 tonne" is mostly marketing

Touring & recovery · 7 min read · Updated 2026-07-27

The brochure says the ute tows three and a half tonnes. You've bought a van near that, loaded the ute with a canopy, drawers, water, fuel, two adults and a dog, and someone at the campground has told you you're illegal. They're probably right, and the reason isn't towing capacity. It's GCM.

Maximum towing capacity is measured in isolation. It tells you the heaviest trailer the vehicle can pull. It does not tell you what the vehicle is allowed to weigh while pulling it, and on most dual cab utes those two limits cannot both be reached at once. Here are the definitions, then the arithmetic, so you can work out your own combination instead of trusting a spec sheet.

The words, in the order you'll need them

Kerb mass is what the vehicle weighs unladen. Be careful: kerb mass, tare mass and unladen mass are used to mean subtly different things — some include a full tank of fuel, some don't, some include a driver, some don't. Never mix a figure from one source with one from another. The only kerb weight worth using is off a weighbridge with your vehicle on it.

GVM — Gross Vehicle Mass — is the maximum the vehicle itself may weigh fully loaded, set by the manufacturer. It's on the identification plate.

Payload is GVM minus what the vehicle actually weighs right now. Not a fixed brochure number — it shrinks with every accessory you bolt on.

GCM — Gross Combination Mass — is the maximum for the vehicle and the trailer added together. Also manufacturer-set, also on the plate or in the manual. This is the limit that catches people.

ATM — Aggregate Trailer Mass — is the maximum the trailer may weigh loaded and standing on its own, uncoupled, jockey wheel down. It's on the trailer's plate.

GTM — Gross Trailer Mass — is the maximum carried by the trailer's own axles when it's coupled to the vehicle. It's ATM minus whatever is resting on the tow ball.

Tow ball download, or ball weight, is the vertical load the coupling puts on the ball. It's the bit that transfers from the trailer to the ute.

Two more limits sit quietly in the background and stop plenty of otherwise legal combinations: axle ratings, front and rear, on the identification plate; and the rated capacity of the towbar and coupling, including a maximum ball mass, stamped or labelled on the hardware itself.

The arithmetic, done properly

Here's the method. Do it once, write the answers on a card, keep it in the glovebox.

1. Collect the manufacturer's limits for your exact variant. GVM, GCM, maximum braked towing capacity, maximum tow ball mass, front and rear axle ratings. Variant matters — cab configuration, drivetrain and even trim can change them. Get them from your own manual and plate, not a review.
2. Weigh the vehicle the way you actually travel. Public weighbridge, full fuel and water, all the gear in, everyone aboard, trailer disconnected. If the weighbridge can give you front and rear axle figures separately, take them.
3. Weigh the trailer loaded. Uncoupled, on its own wheels and jockey wheel, packed for the trip. That's your real ATM.
4. Measure the ball weight. A ball weight scale is cheap and the only honest way. Estimating from how the van sits is not a measurement.
5. Check the vehicle against GVM. Loaded vehicle mass plus ball weight must be at or under GVM. The ball weight becomes part of the vehicle's load the moment you hitch up.
6. Check the rear axle. Ball weight sits behind the rear axle, so it acts on a lever: it adds more than its own value to the rear axle and takes weight off the front. Under GVM and over the rear axle rating is a common, entirely legal-looking situation.
7. Check the combination against GCM. Add loaded vehicle mass, ball weight and the trailer's GTM. Equivalently: loaded vehicle mass plus the full ATM. Both give the same answer, as long as you don't count the ball weight twice — the single most common arithmetic mistake in this whole subject.
8. Check the towbar and coupling. They have their own ratings, and a towbar rated for a given trailer mass usually also states a maximum ball mass.

Every one has to pass. Failing any single one means the combination isn't legal, no matter how comfortable it feels to drive.

Why the maximum tow figure is mostly marketing

Run step 7 on almost any dual cab ute and the pattern appears. Subtract the maximum braked towing capacity from GCM, and what's left is the most the vehicle itself may weigh while towing that trailer. On a lot of vehicles that remainder is well below GVM — sometimes below what the ute weighs once a bar, a canopy and a family are in it, before a single bag of luggage.

That's the trick. Marketing quotes towing capacity because it's the biggest number and it's tested in isolation. The combination limit is the one you live under.

We're not printing figures for specific vehicles, deliberately. They differ by variant and model year, manufacturers revise them, and a number half-remembered from an article is worth nothing at a weighbridge. Do the sum with your own plate and ticket.

Ball weight is not a free number to choose

A common Australian rule of thumb puts tow ball download at around a tenth of the trailer's loaded mass for a conventional caravan. Treat that as a planning figure, not a fact about your van. Actual ball weight depends on how the trailer was built and where the heavy things sit in it — moving full water tanks or a generator from behind the axle to in front of it changes it substantially.

Too little ball weight and the trailer is prone to sway, a genuinely dangerous failure mode. Too much and you overload the rear axle, unweight the front, and lose steering and braking authority at the end you steer with. Measure it, and measure again after you rearrange the packing.

A weight distribution hitch moves some of that download back onto the front axle and the trailer. Useful, and it does not increase any limit — the mass is still in the combination. Some vehicle manufacturers place conditions on their use, particularly around driver-assist systems, so check your manual and the hitch manufacturer's instructions first.

The things that follow from the numbers

• A GVM upgrade doesn't automatically raise GCM. You can gain payload and lose towing headroom. Ask the certifier about GCM explicitly, in writing.
• Accessories compete with ball weight. A steel bar, a winch, a long range tank and a full water tank all come out of the same budget as the download.
• Rating changes have flow-on effects. If an upgrade pushes the vehicle near the mass threshold where it stops being treated as a light vehicle, licence, registration and regulatory obligations can change. Ask your state road authority and the certifier before, not after.
• Trailer braking and safety chain requirements are set by regulation and vary in detail. Check your state or territory's requirements for the mass you're actually towing.
• Insurance. If you're over a limit in a crash, expect that to be examined. Talk to your insurer about your build and your combination.

Things that make the numbers easier to live with

Weigh at the start of a trip, not the end of the planning. Discovering you're over at a weighbridge costs a few dollars. Discovering it at a roadside check costs rather more.

Pack the heavy things low and over or slightly forward of the trailer axles, not in the rear boot of the van where they multiply sway. In the ute, keep weight forward of the rear axle where you can.

And be honest about water. A litre is a kilogram, and travelling with full tanks in both the van and the ute is how a legal combination becomes an illegal one between campsites.

Before you buy

• Get GVM, GCM, maximum braked tow, maximum ball mass and axle ratings for your exact variant from your own plate and manual.
• Weigh the loaded vehicle at a public weighbridge before you commit to a trailer.
• Do the GCM sum, and don't count the ball weight twice.
• Buy a ball weight scale and use it after any change to how the trailer is packed.
• Check the towbar and coupling ratings, including maximum ball mass, suit the trailer you're buying.
• Confirm the retailer lists the towbar and wiring harness for your series and build date, and quote your VIN — towbar fitment is series and body-specific.
• Ask about GCM specifically if you're considering a GVM upgrade.
• Check your state's requirements for trailer brakes and chains at the mass you're towing.
• Tell your insurer what you've fitted and what you tow.

This guide is general information, not fitting or engineering advice. Fitment, compliance and safety depend on your specific vehicle — confirm with the retailer and a qualified fitter before you buy.