Understanding GVM, GCM and payload

Every truck driver and owner-operator needs to understand the three critical weight measurements that determine what you can legally carry and tow. Getting these wrong doesn't just risk fines – it compromises safety and can void your insurance.

Here's what each measurement tells you: 

GVM is the legal maximum weight your loaded vehicle can be on the road. This includes the vehicle itself, fuel, passengers, freight, accessories, and modifications such as bull bars.

The manufacturer sets this limit based on the vehicle's structural capacity, brakes, suspension, and chassis strength.

You'll find your GVM on the compliance plate, usually located on the chassis rail or driver's door jamb. Operating above your GVM is illegal and unsafe, regardless of how capable the vehicle feels. 

Overloading affects braking distance, handling, and puts excessive stress on components that weren't designed for that weight.

GCM is the legal maximum total mass of your prime mover or rigid plus trailer/trailers when both are fully loaded. Like GVM, the manufacturer sets this limit.

A practical example would be if your bogie drive prime mover weighs 8500kg loaded and your GCM is 42,500kg, your tri-axle trailer can weigh a maximum of 34,000kg. You can't simply load the trailer to its maximum capacity if this pushes your combination over GCM.

Both numbers – your current prime mover weight and your current trailer weight – must stay at or under the GCM figure.

Tare weight (sometimes called kerb weight) is your vehicle's weight when empty but with a full tank of fuel and standard fluids.

Payload includes everything you add after that:

That last point is critical for combination vehicles.

When you hitch up a loaded trailer, a significant portion of its weight transfers onto your prime mover through the turntable – a 30-tonne trailer might place 4.5 to 6-tonnes there alone.

A prime mover typically has six to 7-tonnes of payload capacity, but once you account for fuel (up to 400kg for dual tanks), driver and passenger (150-200kg), sleeper gear (100-150kg), and that turntable download, your effective payload shrinks quickly. This is when it pays to weigh before loading.

Understanding axle mass limits is essential for legal compliance and load distribution.

These limits depend on whether axles have single- or dual-tyres and their position on the vehicle.

Single steer axles with single tyres are generally limited to 6.0-tonnes under general mass limits (GML), though some complying steer axles can be rated up to approximately 6.5-tonnes.

Single non-steer axles with single tyres typically max out at 6.0-tonnes at GML, depending on tyre width.

When you fit dual tyres to a single non-steer axle, the limit commonly jumps to around 9.0-tonnes at GML, again subject to tyre size and configuration.

Tandem axle groups show significant variation based on tyre configuration.

A tandem with single tyres on both axles is typically limited to 11 to 14-tonnes at GML depending on tyre size and configuration. Fit dual tyres to both axles and that same tandem group can generally carry up to 16.5-tonnes.

Tri-axle groups follow similar patterns. With single tyres, you're usually looking at 15-20-tonnes at GML depending on tyre width. Tri-axle groups with dual tyres or modern wide-single tyres can reach approximately 20-tonnes.

These figures represent standard General Mass Limit allowances for heavy vehicles. However, several factors can modify these limits:

Understanding these weight limits isn't just about avoiding fines – it's about operating safely within your vehicle's design limits. Before you load freight or hitch up a trailer, do the math.

Check your compliance plate, weigh your vehicle if you're uncertain, and always leave a margin for error.

The weighbridges don't lie. Know your limits, respect them, and you'll protect both your business and everyone else on the road.

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