Guide to GVM upgrades: what it is and where to get one

Got some burning questions about GVM upgrades and need a simple explanation? We cover the what, why, how and who to go with here.

By Dan Everett 15 Min Read

Got some burning questions about GVM upgrades and need a simple explanation? We cover the what, why, how and who to go with here.

Understanding your vehicle’s ability to carry weight is one of the most important considerations when building your dream rig and talking about GVM upgrades. Unfortunately, it’s also one of the most complicated. There’s a sea of acronyms to deal with. GVM, ATM, and GCM are all hard enough to wrap your head around to begin with. Throw in the fact that every single guide on the topic ever written has apparently been aimed at people with a degree in advanced maths, and it’s no wonder the average punter just closes their eyes and hopes for the best. 

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Thankfully, we’re a lovely bunch around these parts. So, we’ve made the most straightforward guide to vehicle weights and GVM upgrades. After a few minutes of reading, you’ll be a Kung-Fu Master of aggregate trailer mass (ATM). You’ll be able to rattle off the who’s, why’s, and what’s of gross vehicle mass (GVM). And you’ll be able to hold a solid conversation on the nuances of gross combined mass (GCM) at your next dinner party. Failing all that, you’ll at least be able to make an informed choice decking your 4X4 out for that trip of a lifetime. 

GVM upgrades
Any GVM upgrade will need to be done by the pros, like the team at Ultimate Suspension

What’s with all the jargon?

I know, right? It’s endless. But hopefully, we can help make heads or tails of them. Once you get a solid grasp of not only what they stand for but what that means, life will be a whole heap easier later on, particularly when it comes to understanding the whys of GVM upgrades.

Kerb weight

Ask ten people what exactly kerb weight means, and you’ll get 11 different answers. So don’t worry too much about its specifics yet. Essentially it means what your 4X4 should weigh without any modifications fitted or people in it. Different interpretations include varying levels of fluids and occasionally with a driver in it. To know what you’re really starting with, your best bet is to run over a weighbridge.

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GVM

Gross vehicle mass. We’ll be discussing this at length. But the short version? It means the absolute total no exceptions maximum your 4X4 is allowed to weigh. If your GVM is legally bang-on, you pop a disgustingly good Snickers bar into your pocket, and you’re now illegally overweight. So yes, this is what you’ll be increasing with a GVM upgrade.

Payload 

Your payload is how much you can load up into your 4X4. Working this out is generally pretty straightforward. Get your GVM – the maximum your 4X4 can legally weigh. Subtract your kerb weight – how much your 4X4 normally weighs. The amount left over is how much extra you can pile on. Your payload will increase with a GVM upgrade.

Axle load

Let’s say you’ve got a dodgy arm. All up, you might be able to lift 40kg. But your dodgy left arm isn’t as strong as your right arm. Out of that 40kg, 25kg might need to go on your right arm and only 15kg on your left. That is axle loads in a nutshell. Rear-axle loads are typically higher than fronts. And fronts will be affected by the extra weight of your engine and gearbox. You can have your axle loads tested at specialised suspension outlets, where scales are used to weigh the front and rear axle loads.

Unbraked towing 

Just like you’ve got a maximum legal weight your 4X4 is allowed to weigh, there’s a maximum you’re allowed to tow as well. Unbraked towing is how heavy your trailer is allowed to be until it needs its own brakes. For most 4X4s, this is going to be 750kg.

Braked towing

There are no bonus points for guessing this one. It’s the maximum weight of a trailer you can tow that has brakes. Simple, eh?

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GCM

GCM is your gross combined mass. That’s the maximum legal limit your 4X4 and trailer combined can weigh. It’s important to know that a GVM upgrade does not change your GCM.

Tare

Think of tare as your trailer’s kerb weight, and you’re home free. It’ll include every accessory fitted from the factory but not things like food, water, or your clothes.

ATM

ATM is your aggregate trailer mass. Think GVM but for your trailer. Just like GVM, this is the maximum it’s ever allowed to weigh. That includes the extra thick quilt set you had to have! Similar to a GVM upgrade, you can actually get an ATM upgrade for your caravan as well.

Ball weight

Your trailer will also have an axle rating, but around five to 10 per cent of the trailer’s total weight will end up on the tow ball.

GVM upgrades
Getting a GVM upgrade approved is more than just filing some paperwork. This brand new Toyota 79 Series has received a GVM upgrade from the pros at Superior Engineering.

That’s great, but what’s the problem?

All of this is great on paper and reasonably easy to understand. The problem is that the margins in some of these areas are paper thin. We’ll keep the maths to a minimum here, but even the absolute workhorse 200 Series LandCruiser just doesn’t add up. A mid-spec GXL has a GVM of 3350kg and a kerb weight of 2740kg. That leaves a payload of just 610kg.

That might sound like a lot, but it doesn’t go that far. Throw on the usual list of accessories like a steel bull bar, a winch, storage drawers, fridge, electrical setup, heavier wheels and tyres and maybe a long-range fuel tank and that 610kg can quickly get down to 0. Oh, you’ll need to take the weight of fuel and passengers out of that 610kg too. It’s not hard to imagine that most 4X4s on the road are over their GVM and why a GVM upgrade is now suddenly so appealing.

Modern dual-cab Utes will typically fare better here. For example, the upcoming Ranger has a payload of 995kg. The extra 385kg might just allow you to fill the tank and load up the family legally. As long as you’re not running a heavy canopy, touring setup, or have a big tow ball weight from your trailer eating into your payload.

Can’t I just ignore the GVM problem and not get an upgrade?

Well yeah, I suppose? But there might be some pretty serious ramifications if you do. The most direct to you will be the adverse effects on your 4X4. Just like a playground see-saw with the local cake-eating aficionado on one end, the salad-eating kid on the other will be sent sky-high. Your 4X4 will do the same thing. It will result in your back end squatting and the front end raising, as well as a decrease in steering ability. You’ll also potentially bend your chassis, your braking will suck, and so will your handling.

There’s a whole host of legal and insurance considerations to factor in too. You’re unlikely to get pinged at a roadside RBT for being 200kg overweight, but if you’re involved in an accident you can bet your bottom dollar your insurance company will leave you high and dry when they realise your braking ability was compromised. Are you starting to see why you might need a GVM upgrade, now?

Take it straight from the horse’s mouth, though.

An overloaded Mitsubishi Triton towing a 79 Series LandCruiser
Photo: Courtesy of NSW Police Force Traffic and Highway Command

You’ve convinced me, but what exactly is a GVM upgrade?

A GVM upgrade is a term for a whole bunch of accessories. Essentially, some very clever people in the aftermarket have sat down with some very stern people from the traffic authorities and worked out exactly what’s stopping your 4X4 from being able to safely carry more weight. In some states, these upgrades can be done to any old 4X4 where a kit is available and legally recognised. Others, however, require installation before the vehicle is registered.

For small weight increases, these kits are typically simple: a set of springs, shocks, and a slip of paper. As the desired weight increases, the complexity of the kits has to increase too. A diff housing that’s fine at 3500kg might not be up to scratch at 4000kg. Heavier-rated GVM upgrade kits will typically feature improved braking systems, heavier-rated wheel and tyre packages, larger indicators due to a heavier weight class, and in some situations, coil conversions. Hey, while you’re in there anyway, right?

GVM upgrades
A GVM Upgrade kit from Terrain tamer

Got it, but why were you talking about trailers?

I thought you’d never ask! Here is where things get frustrating. Or exciting if you’re a masochist … no judgement. We mentioned before you need to factor your towball weight into your payload, but there’s more to consider too. That fancy new GVM upgrade you just got fitted will increase how much your 4X4 can legally weigh, but in almost all cases, it won’t increase your gross combined mass.

If your 4X4 now weighs 500kg more, your trailer must weigh 500kg less. There are some exceptions to this rule and a few options for increasing your braked tow rating. But unless you’re ready to speak to a shop about specifics, we’re just complicating things unnecessarily.

What if I don’t want a GVM upgrade?

For a lot of 4X4 owners, a GVM upgrade simply isn’t an option. Whether it’s a budget concern, a lack of options for their specific 4X4, or wanting to run a specific accessory that won’t work with the kit. Luckily, all hope is not lost and there are a few tricks to keeping weight down.

The biggest weight saver is ditching the steel bar work for aluminium. You’ll give up a little protection, but between a rear bar, front bar, bash plates and a roof rack, you can easily save 150kg. Steel long-range fuel tanks can be swapped out for lightweight poly offerings. All terrains will typically weigh less than mud terrains. Alloy wheels are lighter than steel. Also, lithium power setups will have significant weight savings over old-school AGM offerings.

Lastly, do you really need an absolute monstrosity of a caravan? Even dropping back to a hybrid camper can save 1500kg out of your GCM, and potentially up to 200kg off your towball weight.

Who you gonna call for your GVM upgrade?

Superior Engineering

Superior Engineering has become one of the best-known premium 4×4 suspension and accessory manufacturers and suppliers in not only Australia but around the world. They use only the highest quality materials and expert workmanship to produce premium 4×4 products and results for their customers.

Ph: 07 5433 1411

Terrain Tamer

50 years in the making, Terrain Tamer’s parts have been designed, manufactured and tested to withstand some of the harshest conditions Australia has to offer. When you purchase a Terrain Tamer part, you can be confident that it will stand up to whatever you throw at it.

Ph: 1300 888 444

The Ultimate Suspension

Ultimate Suspension doesn’t believe in a one-size-fits-all approach, offering a custom solution to your GVM upgrade and suspension requirements. Industry leaders since 1975, their reputation has been built on superior knowledge and excellent service.

Ph: 02 9618 7674

ARB

Ph: 1300 272 494

Tough Dog

Ph: +61 2 9672 8899



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