The tow vehicle arithmetic in full

There are four checks. Each one is a subtraction. None of them require a rule of thumb, and none of them can be traded off against each other: a combination is legal only if it passes all four.

What follows is the same arithmetic the CampSpec tow calculator performs, written out.

The six numbers you need

Four come from the vehicle and two from the van.

Figure	Where it comes from
Kerb mass	Vehicle, as delivered with fluids
GVM	Vehicle, maximum for the vehicle alone
GCM	Vehicle, maximum for vehicle and trailer together
Braked towing capacity	Vehicle
ATM	Van, maximum fully loaded on its own wheels
Ball weight	Van, mass carried on the towbar

Add one more number that comes from you: everything you put in the vehicle. People, dogs, fuel above the fill used for the kerb figure, recovery gear, the toolbox in the tub. Two adults at 80 kg each plus 150 kg of gear is 310 kg, and that is the load-out used throughout this guide.

Coverage in our table is uneven and it matters. Of 675 variants, all 675 publish braked capacity, 645 publish a maximum ball load, 641 publish GVM, 600 publish kerb mass and 543 publish GCM. A check with a missing figure is not a pass. It is a check that cannot be run.

Check 1: ATM against braked towing capacity

The van’s ATM must not exceed the vehicle’s braked towing capacity.

This is the check everyone already does, and it is the one least likely to be the binding constraint. It uses the van’s ATM, not its tare, because ATM is the mass the van is permitted to reach.

Check 2: ball weight against maximum ball load

The van’s ball weight must not exceed the vehicle’s maximum ball load, and it must also not exceed the towbar and tow ball rating, which can be lower than the vehicle’s figure.

Published van ball weights are tare-state figures. The median across 599 vans with a published figure is 185 kg, which is 6.5 per cent of ATM. Handbooks commonly specify a minimum nose weight of 10 per cent of the towed mass, so a 3,500 kg van at 10 per cent is 350 kg. If the van’s ball weight is not published, 10 per cent of ATM is the working assumption, and it is the assumption the CampSpec calculator uses.

Check 3: payload after ball load

GVM less kerb mass is the vehicle’s payload. Subtract what you have loaded, then subtract the ball weight. What is left must be zero or more.

payload left = GVM − kerb − your load − ball weight

Ball load is not a separate allowance sitting outside GVM. It is payload, and it is the largest single item most towing vehicles carry.

Across the 561 variants publishing kerb mass, GVM and a ball rating, median payload is 1,041 kg. At 310 kg aboard and a 300 kg ball, the median falls to 431 kg and 40 variants are already over GVM.

Check 4: GCM

vehicle side = kerb + your load
combination  = vehicle side + van ATM
combination must not exceed GCM

The ball weight is deliberately absent from the vehicle side. GCM covers the laden vehicle plus the trailer mass carried on the trailer’s own wheels, and the ball load is already inside the van’s ATM. Counting it twice makes the combination look heavier than it is.

This is the check that fails most often once a vehicle is loaded, and the one no brochure figure warns you about. Of the 341 variants rated at 3,500 kg that publish kerb mass, GVM and GCM, only 38 have a GCM of at least GVM plus 3,500 kg. The median shortfall across the other 303 is 600 kg.

Worked example one: passes on rating, fails on GCM

The van is a Snowy River Caravans SRL-236: 2,813 kg tare, 3,500 kg ATM, 302 kg ball weight.

The vehicle is a Ford Everest Platinum 4WD 3.0 V6: 2,465 kg kerb, 3,150 kg GVM, 6,250 kg GCM, 3,500 kg braked, 350 kg maximum ball load.

Load-out is 310 kg.

Check	Arithmetic	Result
ATM against rating	3,500 against 3,500	At the limit
Ball against rating	302 against 350	48 kg spare
Payload after ball	3,150 − 2,465 − 310 − 302	73 kg left
Combination against GCM	2,465 + 310 + 3,500 = 6,275 against 6,250	Over by 25 kg

Three checks pass, one fails, and the combination is illegal. Twenty-five kilograms is one adult passenger less, or one full jerry can, or the toolbox. Nothing in the brochure signals it, because the brochure shows a 3,500 kg tow rating and a 3,500 kg van.

The working tow limit for this vehicle at this load-out is GCM less kerb less load, which is 3,475 kg. That is the number that governs, not 3,500.

Worked example two: passes everything

Same van. The vehicle is a Toyota LandCruiser GXL Wagon 2.8L turbo diesel, from the 70 Series range: 2,300 kg kerb, 3,510 kg GVM, 7,010 kg GCM, 3,500 kg braked, 350 kg maximum ball load.

Check	Arithmetic	Result
ATM against rating	3,500 against 3,500	At the limit
Ball against rating	302 against 350	48 kg spare
Payload after ball	3,510 − 2,300 − 310 − 302	598 kg left
Combination against GCM	2,300 + 310 + 3,500 = 6,110 against 7,010	900 kg spare

The tow rating is identical on both vehicles. The GCM differs by 760 kg and the GVM by 360 kg, and that is the whole story.

Reading the result

Three things are worth taking from the arithmetic.

The binding check is rarely the tow rating. Across the 475 variants that publish all four vehicle figures, 301 clear every check against a 3,500 kg van at a 350 kg ball and 310 kg aboard. Among the 348 rated at 3,500 kg or more, 19 fail on GCM, 25 fail on ball rating and 9 fail on payload.

Small load changes flip results. On the 357 variants rated at 3,500 kg that publish kerb mass and GVM, a 350 kg ball puts 21 of them over GVM at 310 kg aboard. At a 250 kg ball, none of them are over.

The van’s ATM is the right input, not its tare. Median payload across 799 models publishing both tare and ATM is 620 kg. Planning against tare and then loading the van is how a legal combination becomes an illegal one without anything on the vehicle changing.

Running it on your own numbers

Check a specific vehicle and van runs all four checks on any variant and model pairing and shows which one governs. If you have chosen the van and not the vehicle, find a vehicle that clears the van starts from the ATM and returns every variant that passes, with the margins you want held back in reserve.

Figures here are manufacturer-published; your vehicle’s compliance plates and handbook govern.