The gross combined weight rating of a vehicle is the maximum amount of weight it can carry and tow. This includes the weight of the vehicle, the weight of any passengers and cargo in the vehicle, plus the weight of any trailer and cargo towed behind the vehicle.
Another important figure is the gross vehicle weight rating. The GVWR consists of the curb weight of the vehicle itself, cargo and passengers inside the vehicle, and the tongue weight of any towed trailer. The tongue weight is a fraction of the overall weight of the trailer and cargo, determined by the manufacturer, that actually rests on the towing hitch of the vehicle. This represents the maximum weight that the vehicle's steering, braking and drive train are designed to carry safely.
Overloading a vehicle can be extremely dangerous. More stress on the drive train can lead to reduced acceleration and power, and an overloaded vehicle can be difficult to steer. Overloaded brakes can grow too hot and fail, which is why regulations typically demand that any trailer capable of carrying more than 1,500 pounds must have its own braking system.
Some states require a commercial driver's license for operating any vehicle with a gross combined weight rating of more than 10,000 pounds.