What you need to know when purchasing a tow vehicle
When purchasing a caravan, a tow-vehicle or both, the weight rating placards can all get a little confusing to know what you’re legally allowed to tow or even how to calculate towing capacity.
But it is essential to understand that your setup must not exceed the tow vehicle manufacturer set limit, Gross Combination Mass (GCM), as it is illegal and could void your insurance if things go wrong.
For example, say you are looking at a vehicle that says it can tow up to 3,500kg and whilst the manufacturer tells you that technically, yes, you can tow up to 3,500kg, you may need to compromise on your payload to do so. Big time.
In this blog, we’ll go over the various acronyms and definitions as well as run though some scenarios to help you understand how the towing capacity of your vehicle, the payload and the caravan/trailer capacity affects one another.
That way, when you’re out looking to purchase your next tow-vehicle or caravan, you’re armed with the correct knowledge.
Tow vehicle capacities explained
When looking at the best Tow Vehicle, there are four fundamental numbers you need to be aware of; Kerb Weight, payload, Gross Vehicle Mass or GVM for short, and towing capacity.
What does vehicle Kerb Weight mean?
Kerb weight (KW) is the weight of the vehicle, the driver, oil and a full tank of fuel. Think of it as the vehicle sitting in the driveway of the dealership ready to be driven away. Kerb Weight does NOT include accessories that you or the dealer may have fitted to the vehicle.
Kerb Weight is the vehicle weight with only a full tank of fuel and nothing else
What Payload means
Payload is the total weight you’re allowed to add legally and safely to the vehicle and includes your occupants, luggage, the tow-ball load (also referred to as the Tow Ball Mass (TBM), and any other accessories installed on the vehicle that don’t come stock standard.
How is maximum payload calculated?
You can work the maximum payload out by subtracting the Kerb Weight (KW) from the Gross Vehicle Mass (GVM).
Maximum Payload = GVM - KW.
One factor that is often overlooked is Tow Ball Mass (TBM). When this mass is exerted onto your vehicle, this mass must be included in your vehicle’s payload. In other words, the TBM can eat into your payload, thereby affecting what you can carry including the number of occupants allowed.
Keeping the TBM within 7-10% of the ATM is a good rule of thumb to ensure your vehicle is more stable to avoid caravan sway, but it’s a good idea to check what the vehicle manufacturer recommends.
How to work out tow ball mass
What does Gross Vehicle Mass mean?
Gross Vehicle Mass (GVM) is specified by the vehicle manufacturer and is the maximum allowable weight of the vehicle including all its occupants, luggage, and the tow-ball mass.
So if you want to know how to calculate gross vehicle mass it’s KW + payload = GVM. The GVM is set by the manufacturer and cannot be exceeded.
It is used, along with the Kerb Weight (KW) to calculate the maximum payload.
Maximum Payload = GVM - KW
How to work out your Gross Vehicle Mass
Find out about Towing Capacity
Towing Capacity, also known as braked towing rating, is the maximum weight that your towing vehicle can legally tow and is set by the vehicle manufacturer.
Vehicle manufacturers equate Towing Capacity to the Aggregate Trailer Mass (ATM).
All these ratings, GVM, KW, ATM, GCM (explained below), are limits set by the vehicle manufacturer and cannot be manipulated or changed by you. To be safe and legal on the road they must be adhered to.
Caravan and Trailer Towing Capacities Explained
We have touched on the definitions of caravan and trailer weights, so for the purpose of this exercise, you should be aware of three weight ratings of your trailer/caravan and that’s, Tare Mass, Aggregate Trailer Mass or ATM and the Gross Trailer Mass or GTM.
Calculating Tare Mass
Tare Mass is the weight of the caravan, any accessories fitted, empty water tanks and empty gas cylinders at the time of manufacture. Think of it as the caravan/trailer as you drive it out of the dealer.
What is Aggregate Trailer Mass?
The Aggregate Trailer mass (ATM) is the weight the trailer exerts on the ground, with everything loaded, when standing free, i.e. not hitched to a tow vehicle.
It must not exceed the tow vehicle's rated Towing Capacity.
The ATM includes all the gear you put in it, the accessories you pack on like bikes, the water tanks and gas cylinders full. The ATM also includes the Tow Ball Mass (TBM).
When the van is loaded, it must not exceed the tow vehicles Towing Rating or the trailer/caravan manufacturer's ATM rating.
Calculating ATM
Gross Trailer Mass definition
Gross Trailer Mass (GTM) can simply be thought as ATM – TBM and when the caravan/trailer is hitched to the tow vehicle, the load through the trailer wheels/tyres.
These figures are determined by the trailer/caravan manufacturer and how the trailer is loaded.
How to determine Trailer Mass
Gross Combination Mass explained
The Gross Combined Mass (GCM) is defined by the vehicle manufacturer and is the maximum limit your combined rig (Vehicle and trailer/caravan) can weigh at anytime.
When you add your Vehicle Mass (VM) and your Trailer Mass (TM), the combined figure is your rig's Combination Mass (CM)
CM = VM + TM
The CM of your rig must not exceed at any time the GCM specified by the tow vehicle manufacturer.
VM+TM = CM < GCM
Calculating Combination Mass
If all of this seems a bit confusing, it’s probably because it actually is, which is why it is useful to go to a weighbridge to see if you are in the green. This video below, The Practical Guide to Modern Towing produced by RV Daily is a great introduction to the various acronyms for your vehicle and caravan as well as tips for what you need to do once you arrive at a weighbrige.
Watch the full episode here
What can I actually tow with my vehicle?
Let’s say you have a tow vehicle with a GVM of 3,200 kg, what is the maximum Trailer Mass TM for the caravan? Or in other words what can you Tow?
When working out what you can tow, a simple equation to remember is this GCM – VM = TM.
The GCM, minus the vehicle mass, is what’s left for the trailer mass.
So, let’s put that into practice.
How to work out maximum combination mass
The Vehicle
| Kerb Weight (KW) | 2,250 kg |
| Gross Vehicle Mass (GVM) | 3,200 kg* |
| Gross Combination Mass (GCM) | 6,000 kg |
| Towing Capacity/Aggregate Trailer Mass (ATM) | 3500 kg |
| Payload | 600 kg |
| Tow Ball Mass | 350 kg |
* The vehicle has a KW of 2,250kg and the GVM is 3,200 kg.
This means that you have a maximum payload of 950 kg to play with (3,200 kg – 2,250kg).
Keep in mind that as the TBM affects payload, the actual amount of luggage, and occupants you can carry reduces to 600 kg (950 kg - 350kg).
The Caravan
| Aggregate Trailer Mass (ATM) | 3500 kg |
| Gross Trailer Mass (GTM) | 3150 kg* |
* Remember that we deduct the TBM from ATM to get GTM.
If my Vehicle is at GVM, what can I tow?
| Gross Combination Mass (GCM) | 6,000 kg |
Working backwards from GCM, with the vehicle loaded to GVM, this is what you can tow:
GCM – GVM = TM
6000 kg – 3,200 kg = 2,800 kg
In other words, if your vehicle is at its GVM limit (this includes the kerb weight, the TBM and then whatever is left over for payload), you can only really have a tow vehicle that has a GTM of max 2,800kg.
That is only 80% of what the rated Towing Capacity the vehicle manufacturer states you can legally tow.
What is the maximum vehicle weight I need for my Caravan?
Let’s say you already have the caravan and its ATM is 3,500kg, and it is loaded to that maximum weight, what would be the maximum vehicle mass VM for the tow vehicle?
Or in other words, what is the maximum weight I can put in my vehicle and legally tow a caravan loaded to the ATM?
Running the same numbers as above this is how you would work it out.
How to calculate maximum vehicle weight
Working backwards from GCM this is the maximum vehicle weight:
GCM – GTM = VM
Or 6000kg – 3,150kg = 2,850kg
In other words, if you wanted to tow a caravan that is 3,500kg, your maximum vehicle mass (VM) is now 2,850kg and not 3,200kg as is the GVM stated by the vehicle manufacturer.
However, this new figure affects your payload. So, the new payload is:
Payload = VM – KW
or 2,850kg – 2250 = 600kg
But that’s not all, there is also another portion of the equation to consider and that’s the Tow Ball Mass (TBM) and this figure is what really makes it into a deal breaker for many.
The payload is now 250kg (600kg – 350kg)
For some, 250kg may be plenty, but once you add in a couple of adults, two kids, the family dog, and luggage, you can quickly see it may not be enough to add in that 20L jerry can, roof rack and bulbar you’ve been eyeing.
Bottom line
As you can see, compromises have to be made if you want to achieve one thing over another. Want to tow a 3.5 tonne caravan? No problems, it just means your maximum payload may be significantly reduced. In some cases, depending on the vehicle manufacturer specs, you may be able to tow a 3.5 tonne caravan but only when the tow vehicle is empty.
Want to make the most out of your vehicle’s GVM? That’s ok too, provided that your caravan and vehicle combination does not exceed the tow vehicle's GCM. As with the example demonstrated above, that may mean squashing that dream of towing a 3.5 tonne caravan and opting for something smaller.
At the end of the day, this is a guide only, and your best bet is to speak to the vehicle manufacturer and caravan manufacturer to ensure your combination is legal and safe. In addition, it’s always good to go to a weighbridge location each time you decide to set out because the loads and other circumstances may change every time.
Maximising GVM and GTM
