Pancake A brake diaphragm housing on each set of dual wheels on a semi-trailer and on most tractors.

P&D Pickup and delivery.

Payload Weight of the cargo being hauled.

Peddle Run  Truck route with frequent delivery stops.

Pigtail Cable used to transmit electrical power from the tractor to the trailer. So named because it is coiled like a pig's tail.

Piggyback Semi-trailer built with reinforcements to withstand transport by a railroad flatcar.

Pintle Hook Coupling device used in double trailer, triple trailer and truck trailer combinations. It has a curved, fixed towing horn and an upper latch that opens to accept the drawbar eye of a trailer or dolly..

Placard A sign showing the type of hazardous materials loaded in a vehicle. 

Ply Rating (PR) Relative measure of tire casing strength. (see Load Range)

Powertrain (Drivetrain) All the components, including engine, which transmit the engine's power to the rear wheels: clutch, transmission, driveline and drive axle(s)

Private Carrier Business which operates trucks primarily for the purpose of transporting its own products and raw materials. The principle business activity of a private carrier is not transportation.

PSI (Pounds Per Square Inch) In trucking, unit of measurement for tire air pressure, air brake system pressure and turbocharger boost..

PTO (Power Takeoff) Device used to transmit engine power to auxiliary equipment. A PTO often drives a hydraulic pump, which can power a dump body, concrete mixer or refuse packer. Some designs mount to a standard opening on the transmission, while others attach at the front or rear of the engine.

Pull Trailer Short, full trailer (supported by axles front and rear) with an extended tongue. 

Pup Trailer Short trailer, usually between 26 and 32 feet long, with a single axle.