You’ve heard these car terms thrown around pretty often – tare mass, kerb weight, GVM – and you may have a general idea that they all refer to how heavy a vehicle is. That’s correct, but are all these words the same?
Since these terms apply to your vehicle, too, they’re worth learning about to ensure that you are loading or towing within safe limits.
Common Combination Words
Before we get into each of these measures of weight, it would be a good idea to clarify three words – gross, weight, and mass – which are commonly combined to form the different terminologies.
Gross means the totality of something, so the gross weight of canned milk is the sum of the weights of the content and the can.
Weight and mass are technically different. Weight measures the force of gravity on an object, while mass is the measure of the amount of matter in an object. In the car industry, however, it’s safe to use weight and mass interchangeably. So here on, we use both mass and weight to mean the same thing.
With that out of the way, let us now enrich our vocabulary and see through these slightly-confusing weight descriptions.
Different Vehicle Weights & Loads
We will start with the unladen weight of a vehicle.
Tare Mass or Tare Weight
This term refers to the weight of a vehicle inclusive of oils, coolants, and 10 litres of fuel in the tank and without occupants, luggage, bull bars, roof racks, winches, and other aftermarket accessories. It is also called the unladen weight of your vehicle.
Kerb Mass or Kerb Weight
This term is equal to the tare mass or weight but with a full tank of fuel.
Gross Vehicle Mass (GVW) or Gross Vehicle Weight (GVW)
The GVM (or GVW) is the maximum allowable weight that a car is designed for, and must not be exceeded. It is also called the gross plated weight because the manufacturer lists this information along with the VIN on the vehicle’s chassis plate. You may also find this info in the owner’s manual.
This measure of weight is equal to the kerb mass plus the payload.
GVM = Kerb Mass + Payload
- Where Payload is the total permissible weight that your car can carry in addition to its own (kerb weight). This is not just the luggage and stuff you load up your vehicle but includes the weight of all accessories (bull bar, roof racks, others), the driver and all the passengers.
To illustrate this equation, let’s take this example:
If your Toyota Hilux 3.0D has a GVM of 3,000 kg and a kerb weight of 2,000 kg, it means that you can load it up to 1,000 kg.
Loading it beyond 1,000 kg makes it overloaded and not fit to be driven. Remember also that the weight of the driver and other occupants must all be factored into the payload.
Towing Weights & Loads
Now it’s another story if you’re towing something behind your car. Let us learn more about towing weight jargon.
Tare Trailer Mass or Tare Trailer Weight
Tare Trailer Mass refers to the unladen weight of an empty trailer or caravan. Also called the Dry Weight, it excludes the weight of toilet systems, water tanks, and LPG tanks.
Gross Trailer Mass (GTM) or Gross Trailer Weight (GTW)
The GTM is the manufacturer-specified allowable axle load of a trailer (trailer weight plus payload) when it is coupled to a towing vehicle. You can find the GTM on the trailer plate or in the manual.
Aggregate Trailer Mass (ATM) or Aggregate Trailer Weight (ATW)
The ATM is similar to the GTM in the sense that it is the manufacturer-specified maximum weight of a trailer (trailer weight plus payload) except that it is not hitched to a towing vehicle. Between a trailer’s GTM and ATM, the GTM will be the lesser number since a part of the trailer’s weight is supported by the towing vehicle.
ATM = GTM + Tow Ball Download
- Where Tow Ball Download (TBD) is that part of the fully-loaded trailer’s weight that is transferred to the tow bar of the towing vehicle.
Gross Combination Mass (GCM)
The GCM is the total allowable combined weight for both the tow vehicle and the trailer. The vehicle manufacturer sets this maximum limit. Synonymous to GCM are Gross Combined Weight Rating (GCWR), Gross Train Weight (GTW), and Maximum Authorised Mass (MAM).
GCM = ATM + GVM
The Hilux illustration above has a Kerb Weight of 2,000 kg, GVM of 3,000 kg, and Payload of 1,000 kg. Assuming that it is rated with a GCM of 5,000 kg, let’s see the maximum trailer ATM that it can safely tow.
At the Hilux’s kerb weight of 2,000 kg, it’s easy to see that it can tow up to 3,000 kg. However, as you modify it with aftermarket car parts and load it up with supplies and passengers, the weight it can efficiently pull will decrease correspondingly.
Fully loading this Hilux to its GVM of 3,000 kg would reduce its ability to tow a heavy trailer safely. In this case, you would need to lighten the trailer’s weight to 2,000 kg. Alternatively, you may also remove some of the weight on the vehicle if you need to fully load a trailer with an ATM of 3,000 kg to keep within the GCM limit.
To safely load a vehicle, other factors come into play. For instance, the capacity of the tow bar and the trailer type can affect the towing capacity of a vehicle.
For guides and informative articles similar to this one, feel free to explore our website. We also have a Carpedia section and an auto part locating tool that makes searching for car parts convenient and fast.