The best means of determining the towing capacity of a Peterbilt truck is to refer to the owner's manual of the truck. An owner's manual generally provides detailed instructions, specifications and tips for safe towing. If an owner's manual is not available due to loss or theft, Peterbilt has an online resource library that allows truck owners to download owner's manuals specific to their vehicles.
The towing capacity numbers stated in a truck owner's manual are the result of extensive real-world testing performed by professional truck engineers in the most extreme conditions possible. In these tests, the trucks are pushed to the limits to ensure that their towing capabilities measure up to their published ratings. It is never wise to go beyond a truck's towing capacity rating, because overloading a truck can lead to mechanical and maneuverability issues that put the driver, passengers and others at risk.
Aside from towing capacity, there are other crucial weight limits that a driver should never exceed while towing. These include gross vehicle weight, which is the truck's maximum carrying capacity including luggage, fuel passengers and the weight of the vehicle itself; gross axle weight, or the maximum weight that the front and rear wheels can carry; and tongue weight, which refers to the downward force applied by the tongue of the trailer to the hitch of the tow vehicle.