GVM upgrades
The car may carry more, and the towing limit does not change.
GVM is Gross Vehicle Mass, the most your car may weigh loaded, and an upgrade raises that figure. Your towing limit comes from three other figures: GCM, the Gross Combination Mass, the car's tow rating and its towball limit. Only a certificate that also raises those lets you tow a heavier van.
Updated . 9 min read • Technical Guide
A GVM upgrade raises what the car may weigh loaded; it raises your towing limit only if the certificate raises GCM and 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 the one 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 actually buying
GVM stands for Gross Vehicle Mass. It is the most your car may legally 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 actually 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. No certificate, no upgrade, whatever is bolted underneath.
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. Plenty of popular tow cars are at or over 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 car and van added together
- The braked towing capacity
- The towbar's own rating
- The car's towball download limit
- The caravan's ATM, which never had anything to do with the car
The braked towing capacity is the car maker's rating. An aftermarket upgrade does not revise it.
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. Whether it moves is decided by the certification for that vehicle and that package, so 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 three numbers.
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.
- 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 and the towbar are rated to carry on the ball, against 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 3,300 kg, GCM 6,800 kg, tow rating 3,500 kg
- Loaded car, ball weight on: 3,400 kg. Over GVM by 100 kg.
- GCM minus loaded car: 6,800 - 3,400 = 3,400 kg left for the van
- The car is now legal at 3,400 kg, and you may load it to 3,800 kg
- Load it to 3,600 kg: 6,800 - 3,600 = 3,200 kg left for the van
- The van allowance just went down by 200 kg
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.
It is also how a car and a van can each be inside every one of their own limits and the pair still be 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 and solves a real problem. The van's ball weight pressing down on the towbar is usually what put the car over in the first place.
If your problem is that 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 those three limits against a van's published ATM and ball weight, and 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.
Which one applies to your car is a question for a licensed vehicle engineer or approved signatory, working to the rules the road authority in your state or territory sets.
What only an engineer can tell you
Whether a particular upgrade is available, permitted or certifiable for your car where you live is a certification question. The engineer or signatory who would sign it answers that, working to your road authority's rules.
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 documents that carry the new figures, and where you keep them.
- 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 every range page on this site as a published ATM and ball weight, and the terms behind all of it are set out in caravan weights explained.
GVM upgrade questions, answered
Will a GVM upgrade let me tow a heavier caravan?
Usually not. What a caravan may weigh is limited by the car's braked towing capacity, by its towball limit, and by GCM minus whatever the loaded car actually weighs. A GVM upgrade changes what the car itself may weigh, which is a fourth figure. Unless the certification for your particular upgrade also states a higher GCM, and the tow rating still covers the van, the heaviest van you may legally tow has not changed. Get the new figures in writing and run those three sums on them.
Does a GVM upgrade raise my GCM as well?
That is decided by the certification for your vehicle and your package, so it is the first question to ask. GVM is the loaded car. GCM is the loaded car and the loaded caravan together. Raising the first does not automatically raise the second. If GCM is unchanged, the extra capacity you have bought sits inside a combined total that has not moved, so 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?
The words describe when the work is done relative to the vehicle's first registration: pre-rego before it, post-rego after it. They are not the same transaction. The certification route, the evidence required and how the new figures are recorded can each differ between them, and those rules are set by the road authority in your state or territory rather than by the workshop doing the work. Which one applies to your car, and what it involves, is a question for a licensed vehicle engineer or approved signatory before you commit to anything.
Does a GVM upgrade change my towbar rating or ball weight limit?
No. The towbar carries its own rating, and the car has its own towball download limit. Neither figure comes from GVM, so neither rises with it. If the certification for your upgrade states new ones, those are what you go by. If it does not mention them, treat them as unchanged, and ask the workshop for both figures in writing.
Who can certify a GVM upgrade?
By role, a licensed vehicle engineer or an approved signatory, working to the rules the road authority in your state or territory sets. Which people hold that approval, and what they are permitted to sign, is published by that authority rather than by the workshop quoting you. Any workshop quoting the job should be able to tell you exactly who will certify it, what figures the certificate will carry, and what paperwork you keep at the end. Ask for those three things before you pay a deposit.
Run your own figures through it
The tow calculator takes your car's published ratings and a caravan's ATM and works the three limits out together, so you can see which one decides your answer.