by Collyn Rivers
The Gross Combination Mass (GCM) is a rating that specifies the legal maximum weight of the fully laden tow vehicle and the fully laden weight of the trailer. The GCM rating is set by the vehicle manufacture. For most of the more popular tow vehicles it is typically 5400 kg to 6000 kg.
Prior to June 2018, a number of second stage manufacturers (i.e. workshops modifying vehicles with engineered suspension and tow kits) provided kits intended to increase both the vehicle’s Gross Vehicle Mass (the maximum a powered vehicle can weigh when not towing) and also the GCM. The main market was people seeking to tow heavy caravans by vehicles such as the Ford Ranger, Toyota HiLux, Toyota Landcruiser, and the Isuzu D-Max and MU-X.
In June 2018 legislation was passed that prevented from making changes that precluded increasing a vehicle’s original manufacturer-specified (GCM). This legislation was not, however, retrospective. This resulted in a number of vehicles still having having suspension modifications that enables them to legally tow a heavier load than originally stipulated by the manufacturer.
RV Books has mixed feelings regarding this - not least that many owners misunderstand the industry term 'towing capacity'.
Many tow vehicle manufacturers claim a ‘towing capacity’ of 3500 kg. But that claim does not typically advise that this primarily relates to that towable on the end of a rope. It specifies, for example, the ability to stop and restart on a specified gradient.
For most of the more popular dual-cab utes, if a 3500 kg caravan were to be towed, the existing GCM drastically limits that tow vehicle’s payload – to the extent that vehicle is virtually unusable.
If the GCM were to be lifted to enable that tow vehicle’s GVM to be increased, and that increase used only to carry a higher payload, there seems every argument for doing so. If however a GCM increase were to be used to enable still lightly laden utes to tow even heavier caravans that will inevitably further prejudice already marginal towing stability.
The upgrade industry is seemingly being responsible regarding this. To RV Books knowledge, it recognises that increasing GCM (if misused) can prejudice suspension and handling.
There are however a number of pre-2018 vehicles that have GCM upgrades being used tow heavy caravans by vehicles such as the Ford Ranger, Toyota HiLux, Toyota Landcruiser, and the Isuzu D-Max and MU-X. This is not a clever thing to do.
In practice, the maximum towable with such vehicles whilst retaining a reasonable margin of safety (and usable tow vehicle payload) is about 3000 kg. RV Books and many working in this area (as well as the Caravan Council of Australia) strongly recommend not to tow any laden trailer that is heavier than the laden tow vehicle. It prejudices handling and reduces the rig's safe maximum speed.