Transport runs on weight terminology, and most of the industry’s expensive mistakes come from two abbreviations that sound interchangeable and are not: GVW and GTW. The government’s overloading guidance names this confusion directly - mixing up gross vehicle weight with gross train weight is one of the classic routes into an overloading offence. Add MAM, unladen weight, mass in running order and down-plating to the mix and even experienced staff start guessing.
This guide pins down each term using the official definitions, shows where the numbers live on your vehicle, and explains the errors that actually get operators prosecuted.
Start with the government’s vehicle weights guidance, which defines the terms used on plates, in the regulations and in enforcement paperwork.
Unladen weight is the weight of the vehicle when it is not carrying any passengers, goods or other items. It includes the body and all parts normally used with the vehicle or trailer when it is used on a road, but it does not include fuel, and it does not include the batteries in an electric vehicle (with a narrow exception for mobility scooters and powered wheelchairs).
Mass in running order is the total weight of the vehicle when it is empty and ready to be used on the road, without passengers, goods or other items. For cars, buses and lorries it includes the driver, the body and all normally fitted parts, plus essential fluids like oil and a full tank of fuel.
Maximum authorised mass (MAM) is the weight of a vehicle or trailer including the maximum load that can be carried safely when it is being used on the road. This is the term that decides driving licence categories - and it is also known as gross vehicle weight (GVW) or permissible maximum weight. You will find it in the owner’s manual and normally on a plate or sticker fitted to the vehicle.
Gross train weight (GTW) - sometimes called gross combination weight (GCW) - may also appear on that plate or sticker. It is the total weight of the tractor unit plus trailer plus load.
Here is the failure the official guidance warns about word for word: be careful not to mix up the gross vehicle weight with the gross train weight. The gross vehicle weight is the maximum permitted weight of the vehicle plus any load it is carrying. The gross train weight is the maximum permitted weight of the vehicle plus any trailer being towed.
Why does the confusion matter so much? Because the two figures anchor different limits. A rigid truck with a GTW on its plate has headroom in that figure that belongs to a trailer it may not be pulling today. Load the truck itself to its GTW and it is massively overloaded on GVW - illegal, uninsured in the event of a crash, and unstable. The same applies at combination level: the tractor unit has its own GVW, the trailer has its own MAM, and the combination has the GTW. All three limits apply simultaneously, all the time, and the load must fit inside every one of them.
The overloading guidance adds another layer people forget: the gross vehicle weight and gross train weight include the weight of everything being carried - and for GTW, the weight of the trailer itself. Payload is what is left after the vehicle, the trailer, the fuel, the driver and the equipment, not the headline number on the plate.
The maximum permitted axle weight and gross vehicle weight for your vehicle are on a manufacturer’s plate or a Department for Transport plate - usually in the cab, under the bonnet, or on the trailer chassis. When a vehicle carries both, the DfT limits are the legal limits in England, Scotland and Wales.
There are two plating systems behind those plates. Manufacturers’ plating shows the weights the vehicle was designed for - maximum axle, gross and train weights. Ministry plating - the DVSA-issued plate or plating certificate for goods vehicles above 3,500kg gross - denotes the potential maximum legal weight of the vehicle. Neither may be exceeded, and exceeding any of them - axle, gross or train - is a separate offence.
One term in the guidance deserves wider understanding: down-plating. If a vehicle is unlikely to be used at its potential maximum weight, it may be down-plated - meaning a lower weight is shown on the plate or sticker attached to the vehicle. Fleets do this deliberately, for example to keep a vehicle within a lower licence category, to fit drivers’ licence entitlements, or to match the work the vehicle actually does.
Two cautions follow. First, the down-plated figure is now the legal limit - the vehicle’s physical capability is irrelevant, and loading beyond the plate is overloading. Second, anyone buying a used vehicle should check whether it has been down-plated before assuming it can carry what its size suggests.
Weight compliance reduces to a handful of habits built on these definitions. Know the GVW, GTW and axle limits of every vehicle and trailer from their plates - the DfT plate where both exist. Make sure loaders and planners work to GVW for the vehicle and GTW for the combination, never confusing the two. Remember the guidance’s warning not to automatically trust declared weights, invoices or delivery notes from customers - the responsibility for not overloading sits with the operator. Weigh where it matters: you are allowed to drive to the nearest available weighbridge to check your load and then to a place where some of it can be removed if the vehicle proves overloaded.
Get the terminology right and the rest of weight compliance is arithmetic. Get it wrong - one figure mistaken for another on one busy Friday - and you are explaining the difference between GVW and GTW to a Traffic Commissioner instead.