Power density is a measure of the power output of a drive system relative to its weight.

A powertrain with a high power density has a high power capability but weighs little. A system with a lower power density is heavier, or less powerful, or both.

See also energy density.


In motor racing the pit box is the area where the car stops during the race for tyre changes, refuelling (where permitted) and other in-race work.

There is no 'box' as such: the pit box is simply designated by paint or tape markings.

Pit box is often abbreviated to box, particularly on pit boards.

PCM Porsche Communication Management

PDC Parking distance control

PEB Predictive emergency braking

PGM-FI ProGraMmed Fuel Injection (Honda)

PHEV Plug-in hybrid electric vehicle

PHT Parallel Hybrid Truck (GM)

PSA Peugeot Societe Anonyme, parent company of Peugeot and Citroen

PWM Pulse width modulation


In Formula 1 racing, the prime tyre is the harder of the two tyre compounds available to the teams during a race weekend.

Because prime tyres have a softer, grippier compound they generally last longer than option tyres, but do not produce lap times which are as fast. However, the effect varies with the track layout and temperature.

During a dry-weather race, drivers are required to use both prime and option tyres. The rule does not apply in wet-weather races, when wet tyres are used.

See also option tyre


Predictive Emergency Braking (PEB) is a safety system which aims to prevent rear-end collisions.

PEB senses when a collision is becoming likely. It first informs the driver with a warning chime, then if the driver does not intervene the system automatically applies heavy braking to avoid or minimise a collision. PEB deliberately uses as much of the remaining braking distance as possible, rather than stopping the car in the smallest possible space, to give more space for following vehicles to stop.

The video above demonstrates the Bosch adaptive cruise control and predictive emergency braking on the Audi A6.