Gross Combination Mass (GCM): What does it mean?

Ian Bellert — 24 August 2023
Ian Bellert dives into the world of Gross Combination Mass (GCM) to provide readers with an overview as it stands today.

In March 2023, Queensland adopted the LS16 modification code specifying arrangements for re-rating of the Gross Combination Mass (GCM) and Braked Towing Capacity (BTC) of a light vehicle with a Gross Vehicle Mass (GVM) rating that does not exceed 4500kg. The LS16 code details the maximum weight a vehicle can both carry and tow. This is aimed at providing assurance and clarity for consumers and aftermarket manufacturers. The new rules allow heavier loads and bigger combined weights.

Firstly, let's look at the definitions of GCM, GVM and BTC.

Gross Combination Mass (GCM) 

GCM is the maximum loaded weight of the towing vehicle and any trailer, or trailers, being towed while driving on the road. GCM is set by the vehicle’s manufacturer and is the maximum combined weight of the vehicle plus any caravan in tow. Both vehicle and caravan are loaded and the weights are added together to provide the GCM.

Gross Vehicle Mass (GVM) 

GVM is the maximum loaded weight of a vehicle while driving on the road. The vehicle manufacturer states the maximum weight the vehicle can legally weigh when fully loaded. This can be found on the vehicle's compliance plate.

Braked Towing Capacity (BTC) 

This is the capacity your vehicle can tow if your trailer has its own brakes and is determined by the manufacturer of your vehicle. There is a substantial difference between braked towing capacity and unbraked towing capacity. Unbraked trailers are legally restricted to 750kg. 

What is the new code? 

As defined by the Queensland Code of Practice: Vehicle Modifications (QCOP), the LS16 modification code specifies arrangements for re-rating of the Gross Combination Mass (GCM) and Braked Towing Capacity (BTC) of a light vehicle. 

Changes include the upgraded sum of the maximum weight a vehicle can both carry and tow, which will provide assurance and clarity for consumers and aftermarket manufacturers. Also updated was the re-rating of the Gross Vehicle Mass (GVM) which refers to the maximum weight of the vehicle when fully loaded.

Modifications that may be certified under LS16 code are: 

  • GCM rating changes for a vehicle that has an alternative specification by the OEM
  • GCM rating changes for a vehicle that has been inspected and confirmed to conform to a design certificate issued by an appropriately qualified AP
  • GCM rating changes in accordance with the following: 
    • Retaining the OEM GVM and re-rating the Braked Towing Capacity (BTC) of an appropriately approved GVM re-rating, without changing the OEM BTC
    • An appropriately approved GVM re-rating and an appropriately approved BTC rating
    • Re-rating of towing capacity 
  • Restoring the GCM rating to the OEM rating but only after ensuring that all vehicle components are also restored to the OEM specification.

LS16 Code Summary 

The Queensland Code of Practice (QCOP) is intended to supplement the recommendations of the original vehicle manufacturer in relation to vehicle modification techniques or standards and to provide guidelines where vehicle manufacturer’s specifications do not exist. You can download the handbook for QCOP here

What does the new code mean? We spoke with Lesley Yates, Director of Government Relations and Advocacy at the Australian Automotive Aftermarket Association (AAAA), to lift the hood on the changes and what it means to vehicle owners.

“One of the issues that we worked with TMR (QLD Transport and Main Roads) on this code was the effect of a GVM upgrade on GCM. As a vehicle's GVM increases the GCM still has a legal limit to remain compliant,” Lesley said. 

For instance, a GVM upgrade on a 200 Series LandCruiser can increase its GVM from 3350kg to 3800kg. The vehicle and caravan still have a GCM capacity of 6850kg. The original GVM of 3350kg and maximum ATM 3500kg gives a GCM capacity of 6850kg. Adding the GVM upgrade means something has to give in terms of GCM. That means less load somewhere (either tow vehicle or caravan) to remain within the manufacturer's recommendation. A GVM increase does not change the overall GCM. 

Lesley continues: “The AAAA has been involved with the GCM Technical Working Group, an initiative led by the AAAA where industry engineers collaborate to design suitable test protocols and proposed industry standards, as a starting point for a dialogue with state regulators. 

“The AAAA has done a lot in regards to GCM definitions and independent testing vehicle modifications. Our own testing has seen products heat and stress tested which involves the vehicle’s drive-train, transmission and electronic stability control components. Our reports are available as a resource for installers to refer to if engineers require test data before approving the upgrades.” 

The LS16 code demands all suspension kits be comprehensively tested on vehicles under towing conditions. The testing has a number of elements including dynamic performance testing, power-train stress testing and the like. Vehicle testing is designed to ensure that changes to a vehicle via a GCM upgrade, do not affect the vehicle's performance or cause undue stress to the main components used under tow — transmission, driveline and braking systems. 

The LS16 code is very in depth. Second tier manufacturers (makers of aftermarket components) must have their suspension kits validated. That process is very extensive and not cheap. The manufacturer has to supply the ‘evidence pack’ (test data) relating to all the work they have done in getting that kit approved. The evidence pack needs to be supplied to the installer, if an engineer inspecting the installation requires the test data evidence. 

Luke Truskinger, Managing Director, Auto Innovation Centre, spoke with Caravan World

Luke tells us the AAAA has led the industry to develop a process on how to validate a modified GCM. 

“A vehicle’s GCM is listed on most compliance plates of current vehicles designed for towing heavier loads,” Luke said. “Our work is to develop a pathway so registered vehicles with a modified GCM are compliant to the LS16 code. We hope the code will be adopted through other states as well.” 

The hard work is being done by the industry-led body (GCM Technical Working Group) working with state regulators to implement a ‘legal’ standard when it comes to increasing your GCM. It is hoped the LS16 code (or an amended version) is adopted nationally. The hold up is trying to get all parties to agree on the makeup of that final piece of legislation.

Lesley Yates (AAAA) continues with this commentary: “To say there’s confusion in the ranks is on point. The initiative out of QLD Transport and the AAAA’s subsequent involvement has created steps in the right direction to make sure any vehicle modifications have compliance to the code. It's now up to each state to see when, how and if they adopt the code. There is a wait and see approach adopted by some states. We just hope that this great initiative is adopted nationally.”

Caravan World spoke to second tier manufacturers in the suspension sector. The overwhelming view is that a Commonwealth, adopted by all states, can't come quick enough. This will take the guesswork out of GCM compliance and how upgrades to both pre-registration (which can be authorised by a second tier manufacturer) and post-registration upgrades (which have to be authorised by a third party engineer) work. 

What is each state's current view relating to GCM?

According to the AAAA, this is the latest for each state.

For in-service/post registration vehicles (State Compliance) WA, ACT and NT no longer recognise a revised GCM over Original Equipment

Qld, NSW, SA and Vic recognise a revised GCM and Towing Capacity upgrade if endorsed by the inspecting engineering signatory

Tas recognises a revised GCM if endorsed by the inspecting engineering signatory

WA, NT and Tas no longer allow Towing Capacity over Original Equipment.

Until a Federally legislated code is administered and enforced there will be confusion for consumers. If you are looking to upgrade your vehicle's GVM, GCM or BTC capacity, the best advice is to make sure you deal with recognised second-tier manufacturers. It's their job to be up to date with the developments. 

“The AAAA recommends that you discuss your requirements with a reputable installer,” is the final advice from the AAAA’s Lesley Yates. “They will know the codes that govern their locality and have necessary contacts if an engineering certification is required. Always ask the installer, what are my legal limits? If they can't give you definitive advice, walk away pronto.”

Ultimately the goal is safer travelling for tourers, with upgrades to load-carrying capacity on vehicles, completed with certified and tested components, and fitted by experts. 

Alternatively, there is an increasing array of vehicles hitting the market which skirt the GCM issue simply by their massive manufacturers towing capacity. US trucks are highly capable straight off the showroom floor. While not everyone's cup-of-tea, they are worth considering as a tow-vehicle. Check out the review of one of the biggest vehicles on the market — the Chevvy Silverado LTZ 2500HD in Caravan World Issue 636. Off the shelf it’ll tow pretty much anything you’d want!

A special thanks to the hardworking team at AAAA for their invaluable contribution to this article. 


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Weighty matters

Operation 200 Part 1: Legal and loud

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Gross combination mass GCM regulations Braked Towing Capacity updates LS16 modification code Vehicle towing capacity Gross vehicle mass (GVM) Towing safety guidelines Vehicle load-carrying capacity


Ian Bellert and supplied