GVM upgrades
The car may carry more, and the tow rating and GCM (Gross Combination Mass) stay the same unless the certificate changes them.
GVM is Gross Vehicle Mass, the most your car may weigh loaded, and an upgrade raises that figure. Your towing limit comes from other figures: GCM, the tow rating, the towbar's rating and the towball limit. A heavier van fits inside the ratings only if the certificate changes the figure that limits you.
Updated . 9 min read • Technical Guide
A GVM upgrade raises what the car may weigh loaded. Your towing limit moves only if the certificate also changes GCM or the tow rating.
If GCM stays where it was, every extra kilo you load into the car comes off what the van may weigh.
- GVM - the loaded car's limit (people, fuel, gear, bullbar, and the van's ball weight pressing on the towbar)
- GCM - car and van together (the car maker's limit, and one of the limits your towing is measured against)
- A GVM upgrade moves the first number - the second moves only if the engineer's certificate says it does
What you are buying
GVM stands for Gross Vehicle Mass. It is the most your car is rated to weigh with everything in it: people, fuel, luggage, the bullbar and the drawers in the back. The caravan's ball weight counts too. Hitch a loaded van and a couple of hundred kilos of it rides on the car.
A GVM upgrade raises that ceiling, and it comes in two parts you pay for together.
The hardware. Heavier springs and matched shock absorbers, sometimes more than that, depending on the vehicle and the kit.
The paperwork. This is the part that changes the number. Stiffer springs on their own do not change what a car is allowed to weigh. The figure changes when a licensed vehicle engineer or approved signatory certifies the vehicle at the new mass and you hold the paperwork that says so. The certificate is what changes the figure.
Why tow cars run out of GVM first
Add a bullbar, a full tank, a fridge, four people and their luggage, then drop a loaded van's ball weight onto the towbar. Some popular tow cars reach their GVM before they leave the driveway. That is the problem an upgrade is built to fix, and it is a real one. It does not fix the towing sum.
Which limits change
A tow rig is living under several weight limits at once, set by different people. A GVM upgrade changes one of them.
Changed by the upgrade
- The car's GVM
- The car's payload, which is just GVM minus what the car weighs empty
- The suspension under it
Not changed unless the certificate says so
- GCM, the most the car and van may weigh together
- The braked towing capacity
- The towbar's own rating
- The car's towball download limit
- The caravan's ATM
The braked towing capacity starts as the car maker's rating. Some pre-registration upgrades change it, so check the paperwork for the figure that applies.
The towbar has its own rating plate, and the car has its own towball download limit. Neither is worked out from GVM, so neither goes up with it.
GCM is the one to ask about. The certification for that vehicle and package decides whether it moves. Ask for the figure the certificate will carry before you agree to anything.
Where an upgrade shows up in the towing sum
The heaviest van you may tow is the lowest of the braked tow rating, the towbar and towball ratings, and GCM minus the loaded car. Separately, the van's ball weight must fit the lowest towball limit.
The three limits, together
- The braked towing capacity. The car maker's rating. You compare it against the van's ATM, which is the loaded figure. The towbar and towball ratings cap it too, where they are lower.
- GCM minus the loaded car. What is left of the combined limit once the car, its people and its gear are on the scales.
- The towball limit. What the car, the towbar and the towball are rated to carry on the ball. It caps the van's actual ball weight.
The middle one is where a GVM upgrade shows up. The sum below uses made-up but typical numbers.
- Car rated GVM 3,300kg, GCM 6,800kg, tow rating 3,500kg
- Loaded car, ball weight on: 3,400kg. Over GVM by 100kg.
- GCM minus loaded car: 6,800 - 3,400 = 3,400kg left for the van
- The car is now inside its GVM at 3,400kg, and you may load it to 3,800kg
- Load it to 3,600kg: 6,800 - 3,600 = 3,200kg left for the van
- The van allowance just went down by 200kg
Round numbers chosen to show the sum. Put your own car's figures in the tow calculator.
Raise GVM and leave GCM where it is, and the car may be heavier inside a total that has not changed. Every kilo you then load into the car comes out of the van's share. The upgrade fixed the over-GVM problem. The van allowance went down.
That is also how a car and a van can each be inside their own limits while the pair is over GCM.
When an upgrade is the right fix
If your car is over its own GVM once the van is hitched and the family is in, an upgrade raises that limit. The van's ball weight pressing down on the towbar is often what puts the car over in the first place.
If GCM leaves too little for the van you want, raising GVM on its own will not free up any of it. Do the sums with your own figures before you spend anything.
Our tow calculator runs the tow rating, towball limit and GCM against a van's published ATM and ball weight. You can start from your own car instead with Can I Tow It?. If your car has been certified at new figures, those are the ones to put in.
Pre-rego and post-rego
You will hear upgrades described as pre-rego or post-rego. The words mean what they say: the work is done either before the vehicle's first registration or after it.
They are not the same transaction. The certification route, the evidence required and the way the new figures are recorded can each differ between them.
A licensed vehicle engineer or approved signatory can tell you which one applies to your car, under your state or territory's rules.
What only an engineer can tell you
The same engineer or signatory answers whether a particular upgrade is available, permitted or certifiable for your car where you live.
A GVM over 4,500kg needs a heavier licence than a car licence. Ask what the new GVM will be, and check your state's licence rules. Ask whether the rear axle rating changes too, since the ball weight lands mostly there.
The questions below are the ones to put to them, in writing.
Six questions to ask in writing
Ask for every one of these in writing.
- The new GVM. The figure your vehicle will be certified at, as it will appear on the paperwork.
- The GCM afterwards. The number, or a plain confirmation that it does not change.
- The tow rating and towball limit afterwards. Both figures, stated.
- Who certifies it. The engineer or signatory by name, and what they are signing.
- What you receive. The certificate and the label or plate fitted to the car, which carry the new figures.
- What your insurer needs. Whether they want to be told, and what they want to see.
Then do the sum yourself with the new figures. The van's side of it is on each range page on this site as published ATM and ball weight where the maker gives them. The terms behind it are set out in caravan weights explained.
GVM upgrade questions, answered
Will a GVM upgrade let me tow a heavier caravan?
Usually not. The tow rating, the towbar and towball ratings and GCM minus the loaded car set what a van may weigh, and GVM is none of those.
Does a GVM upgrade raise my GCM as well?
Only if the certification for your vehicle and package says so, so ask that first. If GCM is unchanged, anything more you carry in the car comes off what the van may weigh.
What is the difference between a pre-rego and a post-rego GVM upgrade?
Pre-rego work is done before the vehicle's first registration and post-rego work after it. The certification route and evidence can differ between them, so ask the engineer or signatory which applies before you commit.
Does a GVM upgrade change my towbar rating or ball weight limit?
Not usually. The towbar rating and the car's towball limit are set separately from GVM. Treat them as unchanged unless the certificate states new ones.
Who can certify a GVM upgrade?
Before first registration, a second-stage manufacturer approved under the national rules does it. After registration, a licensed engineer or approved signatory certifies it under your state's rules. Ask who will certify it, what figures the certificate will carry and what paperwork you keep, before you pay.
Run your own figures through it
The tow calculator takes your car's published ratings and a caravan's ATM and works the tow rating, towball limit and GCM out together. You can see which one decides your answer.