What is GCM when towing a caravan?
The most your loaded car and loaded caravan may weigh together.
GCM is the most your loaded car and loaded caravan are allowed to weigh together, set by the car's manufacturer. Many cars cannot sit at full GVM (Gross Vehicle Mass) and tow their full rated weight at once. Check it whenever the car is loaded for a trip.
Also called Gross Combination Mass.
- Set by - the car's manufacturer, and listed in the owner's manual
- Counts - the loaded car plus the loaded van
- Catches - a car loaded near its GVM, which can reach its GCM before the van reaches the tow rating
What GCM covers
GCM is the limit on the whole rig: the loaded car and the loaded van together.
The car's manufacturer sets it for the car's engine, drivetrain and brakes.
How GCM can cut your towing capacity
On many cars, GCM is lower than GVM plus the braked towing capacity.
Load the car up and the van you can tow gets lighter.
Work out GCM minus what your loaded car weighs.
When that figure is lower than the tow rating, it is your real towing limit.
Where to find the GCM
GCM is in the owner's manual, towing section.
A GCM upgrade's label and certificate carry its new figure.
If you cannot find it, ask the car maker in writing.
The tow calculator runs the GCM sum with your car's figures.
How GCM can bite before the tow rating
These figures are made up and rounded to keep the sums simple.
Take a ute with a 3,500kg towing capacity and a 6,000kg GCM.
Loaded for a trip, the car weighs 3,000kg.
That leaves 3,000kg for the van, which is 500kg under the tow rating.
A van loaded to a 3,300kg ATM (Aggregate Trailer Mass) is inside the tow rating and over the GCM.
Common mistakes with GCM
- Reading only the tow rating - GCM can set a lower real limit
- Weighing the car empty - use the car as you load it for the trip
- Using the van's tare - use its loaded weight, up to its ATM