What you need to know about modifications to increase Gross Vehicle Mass (GVM)


A HOT “TOPIC” currently is MODIFICATIONS TO INCREASE GROSS VEHICLE MASS (GVM) i.e. how much your vehicle can weigh – all up.

Our in-house guru recently had this to say…. “being legal can be difficult and a bit confusing; a GVM increase may be needed? As vehicles (especially 4WD’s) have become heavier and the list of “essential” equipment grows, many owners now find that their vehicles (and caravans but that’s another discussion) are overweight BUT there are ways around this”. 

Typically, newish 4WD wagons have a payload capability of around 500kg to 750kg, but that ‘payload’ includes the weight of the driver (and any passengers), ball weight (from your caravan or trailer), fuel and additional equipment i.e. usually a fridge (filled with food and drinks), second battery, roof rack, bull bar, winch and second spare wheel / tyre and even the tinnie/motor. Little wonder that most vehicles are overloaded…

Best we know is that there are two options;

  1. A Pre-registration GVM increase.

Heavy 4WDs are now common enough to make it worthwhile for aftermarket suspension makers to produce State authority accepted, legal GVM increase kits for popular brand-new vehicles. However, these fitments need to be done to pre-registered / ONLY new vehicles.

  1. A Post-registration GVM increase.

Already-registered vehicles can be granted a GVM increase but this involves consulting with an engineer who has been approved by your state or territory authority to advise on, assess and certify applicable vehicle modifications.

4 Simple things you need to know when touring and or when towing;

  1. Your vehicle manufacturer’s GVM rating – it’s stamped on the vehicles compliance plate.
  2. The maximum permissible trailer weight your vehicle can tow often referred to as Gross Trailer Weight (Braked or un-Braked) usually stated in the vehicles specification sheet or in the owner’s manual plus permissible ball weight.
    1. Your towbar should be rated for at least these figures and be stamped on a plate located on it.
  3. Your vehicle manufacturers Gross Combined Mass (GCM) rating which is how much your vehicle and caravan / trailer can totally weigh all UP.
  4. Now for the tricky bit – the actual weights of your vehicle and caravan / trailer.
    1. Load your vehicle and caravan / trailer as you plan to use it; top up the fuel, water & gas cylinders; stock up the larder; fill the fridge(s); put the family on board with all your normal travelling gear and head for the nearest weighbridge.
    2. Be sure to weigh your whole rig, as well as the individual weights on each axle.
    3. Do a separate weight measurement to obtain the tow ball weight by disconnecting your vehicle and caravan / trailer and placing the dolly/jockey ONLY on the weighbridge.
      1. Using a scale is another way to get this measurement
      2. Alternatively, a bit of adding and subtracting if you weigh your vehicle and van separately will achieve the same outcome


Now where?

**If your vehicles front and rear axle combined weight exceeds the vehicle manufactures GVM then you are in trouble.

**If the combined weight of your vehicles front and rear axles PLUS the weight of your caravan exceeded the vehicle manufactures GCM then you are again in trouble.

**There are other potential impacts like exceeding the caravan manufactures Gross Trailer Mass (GTM) which is the maximum weight that the caravan wheels can collectively impose on the ground.

Now it time to “start shedding some of those precious items like the 8-piece crockery dinner set in favour of a two-piece melamine set etc…

Post any of the above conditions occurring you now need to reduce weight EVERYWHERE and then do the exercise once more.

GVM increase guidelines

There is no margin of error allowed with GVM, GCM or GTM ratings: if your vehicle or caravan or both are overweight it’s illegal and they can be grounded by state or territory authorities or police. There are heavy fines for this offence and insurance companies have been known to reject claims if vehicles are found to be overweight.


If your vehicle weighs-in heavy you may be able to have it assessed for a GVM upgrade, but don’t even think about it unless the brakes and axles are in top condition and there’s no rust or damage anywhere on the body or chassis. As a rough guide, an increase of around 10 percent may be possible through suspension upgrades. An approved engineer will check the manufacturers axle weights and any modifications made to the vehicle, before re-rating or specifying further upgrades etc. With the engineer’s time and IF new hardware is required, the cost could be +/-$3,000.00.

As a rule, you should allow about 10% of what you will pay for any upgrade to cover the engineers cost. Getting an opinion is often a cost effective first step; this is the best way to consider any increase to your GVM.

*we have no formal association with this company BUT if you are Victorian based and looking for options then a call to Mr. Andrew George of Sigma-Tech on +61 (407) 521 565 or andrew.george@sigma-tech.com.au may be helpful.

Be safe, Be legal