GCM upgrades and Towing Capacity pathways in Australia

Start by identifying the real problem

Customers often ask for a GCM upgrade when the controlling issue is actually something else, such as:

• rear axle load;

• insufficient vehicle payload;

• tow-ball download;

• trailer ATM or GTM;

• towbar or coupling rating;

• tyre or wheel capacity;

• an unsuitable load distribution; or

• a combination that cannot use the vehicle's full advertised towing capacity at its current operating mass.

Before selecting a pathway, confirm the recorded ratings and obtain representative vehicle, axle and trailer weights.

GVM, GCM and BTC are separate ratings

• GVM limits the loaded mass of the motor vehicle.

• GCM limits the loaded mass of the motor vehicle and trailer combination.

• BTC limits the braked trailer mass the vehicle is rated to tow, subject to applicable conditions.

The combination must also comply with axle, tyre, wheel, towbar, coupling, ATM and GTM limits.

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An approved GVM increase does not automatically change GCM or BTC. If GCM remains unchanged, using additional vehicle payload can reduce the mass available for the trailer.

New vehicles and in-service vehicles follow different pathways

New vehicles before they are provided to market

Second Stage of Manufacture (SSM) is a federal first-supply pathway for an eligible new base vehicle. The base vehicle must have the relevant Road Vehicle Standards approval, be entered on the Register of Approved Vehicles and still be new and not yet provided to the Australian market.

An SSM approval holder may complete only the changes covered by its approval and conditions. The presence of an SSM GVM package does not establish that the same approval changes GCM or BTC. Check the approval, vehicle variant and final RAV entry.

Vehicles already registered or otherwise in service

Once a vehicle is in service, state or territory administration generally controls the modification, certification and registration-record pathway.

The nationally developed technical codes do not create one uniform administrative approval. The recognised engineer or certifier, required evidence, approval-before-work requirements, inspection, certificate or plate, and registration-authority decision vary by jurisdiction.

Do not assume that a certificate or modification accepted in one jurisdiction will automatically produce the same registration outcome in another.

Heavy vehicle GCM re-rating

For a motor vehicle with GVM greater than 4.5 tonnes, Vehicle Standards Bulletin 6 (VSB 6) provides national technical pathways for common heavy-vehicle modifications.

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• VSB 6 Section S addresses vehicle rating changes. Its listed pathways include:

• S1 - GVM/GCM re-rating;

• S2 - GVM re-rating (design); and

• S3 - GCM re-rating (design).

The code and evidence must match the actual modification. A design approval is not the same as completing installation inspection, certification and registration steps.

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In Heavy Vehicle National Law jurisdictions, an appropriately authorised Approved Vehicle Examiner performs work within their approval scope. Western Australia and the Northern Territory administer heavy-vehicle modifications through their own systems. The NT MVR may use VSB 6 as a technical reference, but the NT is not an NHVR/HVNL administration jurisdiction.

Light vehicle GCM or towing changes

For a motor vehicle with GVM of 4.5 tonnes or less, VSB 14 provides nationally developed technical requirements for many in-service modifications, while each state and territory controls administration.

There is not one VSB 14 statement that makes a GCM or BTC increase available for every light vehicle throughout Australia. Some proposed changes may rely on a jurisdiction-specific pathway, recognised evidence, manufacturer support or another approval mechanism. Some vehicles or proposed ratings may not have a supportable pathway.

The registering authority and an appropriately authorised engineer or signatory should be consulted before parts are purchased or work begins.