Do you know your gross vehicle mass from your gross combination mass?
How about your tare and kerb weights, or your vehicle's payload capacity?
Here are the basics.
This is what you need to know if you're planning on hitting the road with a caravan, boat or horse float behind.
Here’s a question from Larry, who looks like he needs a 2.2-tonne tow platform that’s reliable, and fuel efficient. Doesn’t have to be fully loaded, and won’t be used off-road. But there’s a lot of towing on in its future - up to eight weeks at a time. Larry also needs to know how all the load-limiting manufacturer’s specifications actually work, plus a recommendation on the vehicle.
“I want to buy a 4WD as a daily driver but also to tow a van that could weigh as much as 2115kg. We need a 4WD that will tow the load, that’s also reliable, fuel efficient, good warranty - and doesn’t have to be top-of-the-line. No hardcore off-roading, but we’ll be towing for one or two months at a time.
“Been looking at various specifications: Does a towing capacity of 2500kg include the load in the vehicle, or is it just the implement being towed? And what do you recommend we buy?”
A lot of this is a numbers game - so let’s find out which numbers you need to know, and what they all mean.
Empty, or 'Unladen' Weight
If we start at the beginning, the first thing you need to know is how much the vehicle weighs, empty. There are two terms for this: kerb weight and tare weight, and they mean - almost - the same thing. And the exact definition depends on which manufacturer you ask.
Tare & Kerb Weight
‘Tare’ means really empty - but with water in the radiator, a token amount of fuel, and all the lubricants on board. 'Kerb’ means all the fuel on board and - sometimes - 75kg added for the driver. But manufacturers often write their own tickets on the exact definition of kerb weight (in particular) and adopt all kinds of weird policies and practices. Both terms (kerb and tare) mean - essentially - the vehicle is empty. Technically, tare weight is a bit less than kerb weight, but there’s no universal standard.
Gross Vehicle Mass (GVM) is the total allowed weight of the vehicle - all the passengers, all the fluids, all the equipment: everything. But not the weight of any trailer. It’s specified by the manufacturer. The maximum, all-up weight allowed. GVM for short.
Both the vehicle and the trailer have a GVM - for the vehicle it means the fully loaded vehicle, and for the trailer it means - likewise - fully loaded.
So, for the vehicle, you get your GVM and you subtract your unladen weight (the kerb weight or the tare weight) and that gives you your payload capacity - which is the total weight of people and equipment you can carry without overloading the vehicle.
And sometimes this is not quite as much as you think.
A Toyota LandCruiser Sahara is a big vehicle. It weighs 2.7 tonnes empty. But the GVM is just 3.35 tonnes. So that means the total payload is just 645kg - not a whole lot, considering there are potentially seven people on board, and a suitcase for each jammed in the back.
It all adds up. 645kg divided by seven seats is 90-something kilos apiece - on average, for the people and their luggage in the pimp’s Cadillac of LandCruisers. It’s pretty easy to overload the vehicle - with a tribe in the back and all the gear for a weekend getaway inside and up on the roofrack.
And don't forget the weight of the accessories you might add: the bullbar, driving lights, winch, roofrack, extra spare wheel, jerry can(s) - all those accessories subtract from what you can carry. It’s a zero-sum game. Which is why so many people tow a trailer.
Above: Accessories are payload. 200kg of aftermarket goodies equals 200kg less payload you can carry onboard
There are two tow capacities: a basic one for light trailers without brakes. Usually that’s limited to 750kg worth of trailer - all up, meaning the trailer plus the load it’s carrying. In other words, the GVM of the trailer. Trailers have tare weights (empty) and gross weights (loaded) - just like the vehicle itself. Tow capacity is the gross weight (or GVM) of the trailer.
The heavier of the two tow capacities is the gross weight of trailers with brakes: car trailers, caravans, horse floats, boats … things like that. Once again, that’s for the trailer plus whatever you put in the trailer - total, all-up weight of whatever you’re towing. The GVM. SUV-type vehicles are often rated in the two-tonne ballpark here. Hardcore off-roaders - at least some of them - like the Grand Cherokee, LandCruiser 200, and utes like the Colorado, BT-50 and Ranger - can stretch as high as three-and-a-half tonnes maximum tow capacity.
Above: All these vehicles offer a 3.5-tonne towing capacity (with brakes)
Total Load Limits: GCM
Of course, it might not be a good idea to load up your vehicle to its maximum payload, and then hook up a trailer that’s right at the upper limit of the tow capacity. You’re talking about driving off into the sunset weighing six-and-a-half tonnes. What’s the point in getting away from it all when you’re taking ‘it all’ with you? It’s also unsafe, potentially, which is why they invented a specification called Gross Combination Mass, or GCM.
Let’s look at a Mazda BT-50 4WD ute. It’s got a 3.5-tonne tow capacity and a 3.2-tonne gross vehicle mass. Add them together you get 6.7 tonnes. But the gross combination mass specified by Mazda is six tonnes neat - that’s the maximum weight (all up) of the vehicle, the payload (of people and equipment) and the total weight of the trailer. So something’s got to give.
If you’re hooking up a heavy trailer and packing the vehicle heavily as well, you’ll need to dial down the all-up weight by losing 700kg somewhere - from the trailer or the vehicle, or both.
Interestingly, some vehicles don’t compromise in this way. In that LandCruiser Sahara we mentioned earlier, the Gross Combination Mass is just the GVM plus the maximum tow capacity. No compromise necessary. With the Cruiser, you are allowed to max-out both the vehicle and the tow capacity. Toyota HiLux: Same thing - just add them both together. But the Colorado’s GCM is about 650kg lighter than the sum of the GVM and the maximum tow capacity. Plenty of other vehicle are this way, too. So it really pays to understand your vehicle’s specifications.
This is another thing that can bring you undone. Vehicle manufacturers specify the towball download limit for their vehicles. This is the vertical download imposed upon the towball by the loaded trailer.
So: The download limit is specified by the manufacturer of the vehicle. You need to know that, and the actual load that is imposed by the trailer in its most heavily loaded state - and you can only know that my measurement. Many specialist tow places have a specialised scale for measuring this, or you can do it at a weighbridge (see below).
What can bring you undone is Australia's mentally retarded attitude to trailer design. Most of the rest of the world uses a design spec that imposes five per cent of the trailer's GVM on the towball. For example, 100kg of download for a trailer with a GVM of 2000kg. In Australia we generally get trailers with 10% download (ie 200kg on a 2000kg trailer).
This is a real problem for vehicle manufacturers and owners because - for example - vehicles like the Hyundai Santa Fe and Kia Sorento come with a 2000kg maximum tow capacity and a towball download limit of 100kg. That's fine in most markets around the world, but often it's a problem here - because the 2000kg trailer you buy is likely to impose exactly double the allowed towball download.
Hyundai accommodates this wrinkle (partially) by offering a Genuine Load Assist Kit, which is basically a set of pumped-up rear springs with variable rate, the better to cope with the additional download. The kit increases the allowable towball download to 150kg.
This uniquely Australian preference for 10% download is easy to overlook - and if you do, you could easily overload the vehicle, void your warranty, and significantly increase the risk you face while towing.
The point is: don’t overload the vehicle in any way. There’s a fundamental safety angle here. Overloaded vehicles are unsafe. There’s a regulatory compliance angle as well. You don’t want to get fined for driving overloaded. And, if you crash, and the vehicle is overloaded at the time, your insurance company might not cover you.
Use a weighbridge. There are plenty out there. Hidden in plain sight. They’re just big, accurate bathroom scales you can drive on, basically. Good idea - because how else are you going to know how much your loaded 4WD weighs, and how are you going to know how much the caravan, horse float … whatever … weighs once you’ve loaded them up with all your stuff? Just Google ‘public weighbridge’ in your area.
2. WHEELS & TYRES
If you’re going to spend a lot of money putting a van and 4WD combination together, and especially if you’re going to visit remote areas, get the van built with the same wheels and tyres as the vehicle. That way, you’ll have greater interchangeability of spare tyres - and that boosts your mobility if you’re a long way from the nearest repairer.
Get the trailer set up properly with the vehicle. That’s a specialist job - the download on the towball is especially important if you want the combination to be dynamically stable. And, trust me, you really do want that.
4. GO SLOW
Heavy towing is a real art form. Don’t be in a hurry. You can’t drive with a heavy trailer behind the way you’d drive unladen. If you’re thinking ‘lap record’ … maybe it’s a good idea if you just don’t tow anything.
5. MARGIN OF SAFETY
Be conservative. If your fully loaded trailer weighs 2.5 tonnes, get a vehicle with a 3.5-tonne maximum tow capacity. It’s called a margin of safety.
I suggest Larry should get his backside in a Mazda BT-50 XT dual-cab auto diesel:
- Five-star ANCAP safety ratings
- 400+ Newton-metres in engine output
- 3.5-tonne tow capacity - so if Larry's needs change down the track and he gets a bigger van, or a big boat, he won't need to upgrade
- Six-speed auto gearbox
- 1000kg+ payload capacity (potentially)