What is GVM and payload and how towing affects it

GVM, GCM, ball weight, axle load, ATM – there’s a lot of boring stuff you need to know about when you’re off-roading and towing. So, for those of you new to the scene, we’re going to answer some common questions, starting with: What is GVM and payload? How does towing a camper trailer or caravan affect it? Does it matter if I go over? Will a GVM upgrade fix it? and more.

What is GVM?

For those who aren’t aware, GVM stands for gross vehicle mass – and it refers to the total amount of weight your 4X4 can handle safely and legally. You can find out what your GVM is by looking at your vehicle’s VIN plate. And pretty much everything affects it, from throwing an extra couple of passengers, filling your fuel tank and fitting a winch to running bigger tyres, packing the camping gear, and even towing a camper. It all goes on the GVM.

What is payload and how is it determined?

Payload and GVM are closely related. If you work out how much weight your vehicle has on board when unladen – so let’s say 10L of fuel, a front bar and maybe a set of drawers and fridge in the back – the difference between that weight and the GVM is the payload. It’s basically a measurement of how much weight you can carry.

Let’s take a 200 Series LandCruiser as an example. A few years back, GVM from the factory was 3,350kg and the kerb weight was 2,705kg (VX turbo-diesel model), leaving a payload of 645kg. Seems fairly substantial, right? But bolt on a bullbar and winch (60kg), a rear bar and a second spare (70kg), a set of drawers (60kg), a 60L fridge on a slide (60kg), a rack (30kg), roof top tent (70kg), a couple of passengers (150kg), a water tank (80kg) and a full long-range fuel tank (120kg) and that weight can be reached, and exceeded, very easily. By the way, we’re now at 700kg and thus illegal. And that’s without a dual battery system, ball weight from a trailer, camping accessories or tools and recovery gear. Scary, ain’t it?

Wait …ball weight affects GVM, too? How?

Towing a camper trailer or caravan significantly affects a vehicle’s GVM because the weight of the trailer’s tow ball (ball weight) is added to the vehicle’s overall weight. This means that when you’re towing, your trailer is eating into your payload (how much stuff you can take along).

Still confused? Check out this short clip below taken from our Are You RV Safe Series.

Does it matter if I go over?

Both the short answer and long answer is yes, it matters if you go over your GVM. Exceeding your GVM, especially while towing, is both dangerous and illegal.

When you think about it, this is kinda scary… as not paying attention to your GVM can have some pretty serious ramifications. If you like the idea of risking a bent chassis, cooked brakes, voided insurance, and automatically being at fault in an accident, then skip on over this information. But if you like being on the right side of both the law and physics, then you need to make a cuppa and keep reading.

She’ll be right …. right?

Now, you may think that driving a 4WD gives you a fair bit of wriggle room here, I mean, most of them have ladder chassis and heavy-duty suspension, and in the good old days, this may well have been the case. But there’s been a disturbing trend over the past couple of decades. 4X4s have gotten wider, longer and heavier, but perversely, the amount of weight they can carry has been shrinking. What the …?

To see if we can get to the bottom of this, let’s have a look at some case studies on how easy it is to go over GVM, wether a GVM upgrade might fix it and what we can do to cut weight.

But first – a quick guide to the jargon

KERB WEIGHT – This is essentially the vehicle’s weight when unladen. It often includes a token amount of weight for a partially filled fuel tank and a driver, but it’s essentially the stock weight of a 4X4.

GVM – Gross Vehicle Mass, or the most a 4X4 can weigh. It can vary through models and trim levels but is always included on the VIN plate.

GCM – Gross Combined Mass, the maximum amount of weight both vehicle and trailer can add up to. It should ideally be the GVM and braked tow rating added together, but often it isn’t (just to make things even more confusing).

ATM – Aggregate Trailer Mass. The weight of a trailer. As a side note, the Gross Trailer Mass (GTM) is the ATM minus the amount of ball weight on the vehicle’s tow bar.

BALL WEIGHT – The force (in kilograms) exerted downwards on a vehicle’s tow ball by a trailer.

PAYLOAD – The difference between the GVM and the kerb weight of a vehicle. Effectively, how much weight it can carry. It includes things like fuel, passengers, bar work, tents, awnings, winches, fridges… you name it.

AXLE LOAD – How much weight the front and rear axles can handle safely. The sum of both axle ratings usually adds up to more than the GVM, giving a bit of flexibility as to where the weight is distributed on the vehicle.

BRAKED TOW RATING – How heavy a trailer (with trailer brakes fitted) the vehicle can tow. As we’ll see in this article, tow ratings are to be treated with a healthy degree of scepticism; but we’ll get into that in a bit…

Will a GVM upgrade fix the problem?

You’ll have a larger payload for your vehicle but this doesn’t necessarily mean you will be able to tow more. The most simple explanation is that forking our dollars for a GVM upgrade does not increase the GCM, which is the Gross Combined Mass of what your vehicle and trailer can weigh together.

So if you’ve loaded your 4X4 up to now weighing 500kg more (due to your GVM upgrade), your trailer must now technically weigh 500kg less as the GCM has not changed. There are some exceptions and GCM upgrades are available, however, you should really speak to a shop about this one as it’s not a minor upgrade and potentially, a different vehicle might be better suited for the job.

3,500kg tow rating – not as impressive as they sound

A few short years ago there weren’t too many options on the market if you wanted to tow a serious load behind you. Today however, almost every dual-cab ute is rated to tow an impressive 3,500kg (braked). Sounds pretty good, doesn’t it?

The thing is, this number is an ‘official’ rating and fails to take into consideration Gross Combined Mass (GCM), Aggregate Trailer Mass (ATM) and its effect on your vehicle’s GVM. When it comes to towing, it’s also worth looking at the front and rear axle load ratings or the amount of weight each axle can handle (which, when combined, usually adds up to more than the vehicle’s GVM). Confusing? Yes, it is.

Let’s look at a case study, shall we?

Let’s take a look at a Nissan NP300 Navara ST-X, which came with a GVM of 2,910kg and a GCM of 5,910kg. Assuming you’re towing the maximum 3,500kg trailer, subtracting the trailer weight drops the allowable GCM to 2,410kg (or the maximum the vehicle can now weigh).

Now we factor in the kerb weight of 1,980kg: That gives 430kg left over for occupants, bar work, canopies, clothes, awnings and the dog.

“Sounds do-able,” I hear you say. But let’s not forget the ball weight of your trailer, which would ring in around the 350kg mark (supposedly, but it’s not really the case; which I’ll explain in a sec). Yep, that’s taken off your GVM too, leaving you a scant 139kg to play with… so bring your spouse, but leave the dog at home and don’t forget to unbolt the bullbar before you leave.

But wait, it gets worse…

What if you want to tow when you’re already at GVM? The GCM of 5,910kg has to be reduced by the GVM of 2,910kg, leaving us with a 3,000kg towing ability. Sounds OK, doesn’t it? Nuh-uh… we need to factor in those axle loads, too. The Navara’s combined axle loads add up to 3,020kg (1,320kg front; 1,700kg rear), which is only 110kg higher than the GVM. Seeing our vehicle is already at GVM, that leaves us only 110kg of allowable ball weight (or a trailer weighing approximately 1,100kg) – but even that’s aiming high. Because the tow ball is located a metre or so behind the rear axle, the leverage changes the load on the axle (effectively making it greater) – so the reality is that even your 1,100kg trailer would be pushing the friendship between you and your vehicle.

Without boring you with loads of maths, the table below roughly highlights the effect of this leverage.

Navara ST-X Ball Weight VS GVM

ATM	Towball download	GVM reduction
1,000kg	100kg	130kg
2,000kg	200kg	280kg
3,000kg	300kg	410kg

So with 350kg of ball weight on our GVM Navara with 3,500kg braked towing capacity, you’re well and truly over GCM and putting all sorts of stress on your axles, engine, brakes… everything really. And oh yeah – you’re breaking the law, too.

Moral of the story? While these vehicles can tow that amount on paper, in the real world, you’re more than likely going to end up driving an illegal, dangerous rig – all the while putting a huge strain on your mechanical components. If you need to tow big loads regularly, look for something with a higher GCM and GVM, and the highest torque numbers you can find.

How to cut weight

We’ve already waffled on enough today, but basically, if you’re serious about cutting weight, you’ll need to drop the non-necessities and swap out older accessories for lighter ones.

When filming the RV Safety series, Are You RV Safe?, we put our learners to the test, packing the caravan and having both the vehicle and caravan professionally weighed. See how they went in the full episode below.

Being over GVM – it’s easier than you think

In case you haven’t picked up what we’re putting down here, it’s incredibly easy to exceed your GVM when you’re loaded up for an extended trip through the Outback.

But let’s be honest here – not everyone who goes over GVM is getting busted. But still, it’s definitely something to think about, especially in the event of things going pear-shaped and your insurance company decides it’s not going to cover you. Not to mention the safety side of things.

At the end of the day, you don’t have much to play with here, and keeping an eye on the weights being carted should be part of your packing regime. To cut a long story short, it’s absolutely imperative that you don’t go over your GVM. The consequences are just too heavy (see what I did there?).

Words by Dex Fulton and Jessica Palmer.

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