GVM vs GCM: the difference, and why it costs you payload

The two limits, in one paragraph

GVM, Gross Vehicle Mass, is a limit on your vehicle when it is loaded: vehicle, fuel, accessories, people, gear and tow ball weight. GCM, Gross Combination Mass, is a limit on the whole rig: that same loaded vehicle plus everything on the trailer behind it.

Both apply at once, along with the towing capacity and, on most vehicles, individual axle limits. You are legal only if you are under all of them. It is entirely possible to be under GVM, under the towing capacity, and still illegal on GCM.

The maths that catches everybody

Take a ute with a 3,500kg GVM and a 3,500kg braked towing capacity. Add those together and you get 7,000kg. Now look up its GCM, and on almost every vehicle sold here it will be several hundred kilos short of that figure. The shortfall is not a rounding error. It is the manufacturer telling you that you may have the full GVM or the full trailer, but not both.

The table below runs that sum for every model we hold figures for: GVM plus maximum braked towing, against the published GCM. The last column is what towing at the maximum takes out of your payload before you load a single thing.

Not in the table: Toyota LandCruiser Prado. The manufacturer does not publish a GCM for these, so the sum cannot be done honestly.

The exceptions: Toyota LandCruiser 79 Series, KGM Musso. On these the published GCM is exactly GVM plus the full towing capacity, so towing at the maximum costs no payload at all. Tow ball weight still does.

Then subtract the tow ball weight as well

The GCM shortfall is only half of the cost of towing. The other half is tow ball weight: the download the trailer puts on the back of your vehicle, typically around ten percent of the trailer's mass. On a 3,500kg van that is roughly 350kg, and it counts as payload.

So a large caravan can easily cost you the GCM shortfall plus 350kg of the payload you had left. On a ute with a payload under a tonne, that is most of it, before the family and the fridge.

This is why so many people discover the problem at a weighbridge on the way to a holiday rather than in a showroom. Each individual number looked fine.

ATM, GTM, and which trailer figure to use

Trailers have two figures of their own. ATM, Aggregate Trailer Mass, is the whole trailer including everything in it when it is unhitched and standing on its own jockey wheel. GTM, Gross Trailer Mass, is what the trailer's own axles carry when it is hitched, which is the ATM minus the tow ball weight.

For GCM you want the whole combination, so the ATM is the figure to work with. Using GTM understates your rig by the tow ball weight, and then you count that weight again in the vehicle, which is where the double counting confusion comes from.

In short: the trailer's full mass goes into the GCM sum, and the portion of it sitting on your tow ball also goes into the GVM sum. Both are true at the same time.

Can you upgrade your GCM?

Sometimes, and less often than people hope. A GVM upgrade raises what the vehicle may weigh, but it does not automatically raise the GCM, so on plenty of vehicles an upgrade buys you carrying capacity and no extra towing capacity at all. Some suppliers offer a separate GCM upgrade, availability varies by vehicle, and it is a matter for a certified engineer rather than a forum.

Before spending anything, work out which limit is actually stopping you. People often buy a GVM upgrade when their real constraint was the GCM, or the rear axle.

Common questions