Carrying out GVM and GCM upgrades in WA

2 October 2024  By: Toby Hagon

In May 2024, the Western Australian government announced standards for GCM (gross combination mass) and GVM (gross vehicle mass) upgrades for light vehicles, to streamline the upgrades process and make it easier for consumers to choose a reputable supplier.

The move largely brings WA into line with most other states when it comes to upgrading the carrying capacity of utes and four-wheel drives.

Mike Davison is the general manager of one of Australia’s largest suppliers of GVM and GCM upgrades, Lovells Springs, and says the changes have simplified the approval process. It predominantly impacts GCM upgrades – now allowing them in WA – while streamlining the approval process for GVM upgrades.

“All this has done … is just formalised the approval system for GVM - and then the GCM (regulatory framework) is totally new.”

The new standards were implemented following consultation with industry as well as the Motor Trade Association of WA, the Australian Automotive Aftermarket Association (AAAA) and the Federal Department of Transport.

It means West Australians who want to tow and go adventuring can safely and legally certify their rigs for the fitment of additional equipment, accessories and luggage.

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Demystifying the acronyms

First, the basics. Cars have maximum load ratings stipulated by the manufacturer and two of the most important are the gross vehicle mass (GVM) and gross combination mass (GCM).

GVM is how much a car weighs with everything in it, from people and luggage to fuel and accessories, including roof racks, bull bars, underbody protection and camping equipment.

GCM is similar but also includes a trailer. If you drove the car and trailer over a weighbridge the combination mass is what you’re measuring.

As manufacturers have battled to offer buyers higher payloads and tow capacities, in some instances they are also pushing the boundaries of what the car can carry, to the point where drivers can easily overload a vehicle.

It’s a balance between providing a comfortable car to trundle around the suburbs largely unladen and preparing a car for being heavily laden and towing.

By engineering a GVM or GCM upgrade it can allow owners to legally carry more, from camping gear or caravans to people and gear.

You’ve only got to look at some of the heavily laden off-roaders cruising the country to realise there’s demand for such upgrades.

GVM or GCM?

GVM upgrades are more common, allowing owners to carry more in and on the car. If you want to accessorise your car for carrying more luggage and gear, then it can be an invaluable upgrade.

Upgrading the GVM can in some instances allow a car to tow closer to its rated maximum capacity when you’re also planning to load up the car.

That’s because some cars have very little payload capacity remaining once they’re lugging a heavy trailer. Similarly, the tow capacity can drop if the car is loaded to its maximum. But as with any modifications there’s no one-size-fits-all.

Cameron Wearing is the senior trainer at Australian Driving Institute and also the manager of the Adelaide Hills 4WD Park and says many owners don’t realise what they’re doing with a GVM upgrade.

“They don’t understand that when you do an upgrade … to theoretically carry more in the car, that it does not have an impact on your GCM,” he says.

Breaching that gross combination mass could leave you driving a dangerous and illegal vehicle. That’s where you may also need to consider the GCM and whether that also needs to be upgraded, something that is a lot less common in the aftermarket.

GCM upgrades can only be done once a GVM upgrade has been implemented (often they’re done together). In each case the car is deemed to have undergone a second stage of manufacture and will be fitted with an additional compliance plate outlining the updates and which approved manufacturer performed the changes.

GVM/GCM upgrades come at a cost

GVM and GCM upgrades are commonly used by families, adventurers and grey nomads looking to fit accessories and head off on the big trip. Fleet operators – including mining companies and government departments – also often upgrade the ratings of their cars.

In many instances the upgrades are done before the car is registered, something that simplifies the approval process. Doing it once the car has been registered means it is treated as a used vehicle, even if it hasn’t travelled far.

As such it will require additional inspections, including checking for the fitment of original componentry as well as for wear and tear and rust, which can add up to $1000 to the cost of the changes.

A GVM upgrade can cost upwards of a few thousand dollars, depending on the supplier and manufacturer. GCM upgrades may add a few hundred dollars to that if they’re performed at the same time as the GVM update, or more if it’s done later. For some vehicles, though, the cost may be higher depending on what needs to be done.

More weight changes the way the car drives

Just because your newly upgraded ride is rated to tow 4.5 tonnes doesn’t mean you should tow 4.5 tonnes. You still need to do the calculations on payload to ensure you can carry all that gear.

Adding weight will change the way the car drives. Revised suspension will mean the car responds differently to bumps and corners.

Just as hitching up a trailer or loading up the SUV can change the dynamics and increase braking distances, more kilograms can amplify the result.

If the suspension has been raised, the centre of gravity will be higher, in turn impacting stability and cornering dynamics. Cameron Wearing says heavier rigs require an adjustment in driving.

“[By carrying so much] they’re increasing the overall kinetic energy of the vehicle,” he says.

“The heavier you are and the faster you go, you need to be more and more and more and more involved (in the driving).”

Wearing adds that accounting for the extra weight is critical.

“Back off in not only speed but back off in the distances they give themselves,” is his message to drivers carrying lots of weight.

Upgrades bring new hardware, but not everything is changed

Engineering work for GVM and GCM upgrades determines what needs to be changed on the core vehicle. What is changed varies from car to car.

A GVM upgrade is the most common and typically involves the replacement of hardware. In most instances it involves new suspension components, including springs, shock absorbers, control arms, bushes and some associated accessories.

The result will be firmer suspension, which will impact the ride comfort, especially when there’s not much weight in the vehicle. That’s because it’s typically heavier duty and designed to cope with heavier loads.

Lovells Springs spells out the changes in the new owner’s manual that comes with the upgrade, recommending there’s at least 250kg in the vehicle at all times.

“GVM upgrade kits are designed for constant load carrying and ride quality will be compromised if not fully laden,” the manual states. GCM upgrades involve, as a minimum, an analysis of the core car, including chassis and suspension components, to ensure each component can deal with additional loads. But it can also include hardware changes.

In some cars, for example, the cooling system may need to be improved, including the radiator and transmission cooler, to prevent overheating.

With either a GVM or GCM upgrade the car is recertified as being manufactured and approved by the secondary stage manufacturer (SSM), something formalised with the fitment of a second compliance plate.

Buy a Toyota Prado, for example, and it’s delivered with approval holder status by Toyota Motor Corporation Australia Limited, something listed on the original compliance plate.

Give that Toyota Prado a GVM upgrade with an approved second stage manufacturer though, and the whole car – not just the modified components – has approval holder status. Essentially, it’s now recognised as being manufactured by your approved manufacturer rather than Toyota.

Car warranty considerations

That also means all warranty implications fall back on the company that performed the upgrade rather than the manufacturer, although the vehicle manufacturer will still warrant items that have failed through no fault of the upgrades.

“All second stage manufacturers have to take on the warranty of the complete vehicle,” says Davison, while explaining that if “the GVM upgrade hasn’t affected [a particular component] the original equipment manufacturer will always take on those warranty (claims)”.

If an infotainment screen fails, for example, you could theoretically push back on the fitter of the GVM upgrade, and they would engage the original manufacturer to make the fix.

Alternatively, you could engage directly with the dealer, and you should have no issues with having it addressed.

A Toyota spokesperson told us that “where a warranty claim is made outside the normal terms and conditions, it will be assessed by the dealer on a case-by-case basis”.

The car will be working harder

Carrying heavier loads means the engine, gearbox and brakes will work harder. And Toyota says fitting heavier duty suspension components could have impacts elsewhere in the vehicle.

“These products may impart forces/loads greater than the original design intent and may compromise the vehicle’s longevity and durability,” a spokesperson said. That’s one reason second stage manufacturers typically have more stringent servicing requirements.

If you’re loading the car beyond what the manufacturer had originally stipulated then it will likely need to be checked more often, something outlined in the revised owner’s manual.

That could mean halving the service intervals from, say, 12 months and 20,000km to six months and 10,000km. With Toyota four-wheel drives, the service intervals could be as little as three months and 5000km.

Beware when fitting other accessories

Vehicle engineering is a science and any changes can have consequences. Fitting a bull bar or driving lights, for example, can affect airflow to the radiator.

If the engine is working harder towing a heavier load it could have serious consequences for cooling and drivetrain longevity.

It’s important to check such additions with the second stage manufacturer (SSM) you’re performing the GVM or GCM upgrades with.

The SSM will also typically only comply the wheels and tyres originally fitted to the vehicle or those engineered as part of the upgrade.

If the owner wants to change wheels and tyres it is their responsibility to ensure they are up to the job with appropriate engineering approval.

Notify your insurer

It’s also important to tell your insurance company of the upgrades.

In many instances it may not impact your premium, although it needs to be noted on the policy to ensure compliance and transparency.

If the changes involve reclassifying the vehicle – designated by an alphanumeric code – there can be implications, according to Davison.

“If it changes category … such as with a Ford Ranger, where factory it comes out as an NA category (vehicle) … then goes to NB1 (after an upgrade) … the registration costs are higher because it becomes a goods carrying vehicle.”

A GVM/GCM upgrade may not be necessary

Any modifications to a car come with compromises and it’s no different when upgrading the carrying capacity with a GVM or GCM upgrade.

Obviously there’s the cost. But GVM and GCM upgrades can also negatively impact the comfort and some aspects of the way it drives, especially when unladen.

So if you don’t need to carry more than the car was originally rated to carry (or tow) then you may be going backwards by upgrading suspension and other components to cope with heavier loads.

And while a higher ride height that typically comes with a GVM upgrade can make the car look tougher and improve clearance over obstacles, stiffer springs can make it harder for wheels to droop and compress over rocks, dips and logs.

“It may have clearance but it’s no longer as useable,” says Cameron Wearing from the Australian Driving Institute, urging people to consider what they’re trying to achieve with a GVM upgrade.

“The first question I ask is why…what’s the gain you’re trying to get?”