PCD stands for Pitch Circle Diameter, which is the diameter of the circle that passes through the center of all the studs, wheel bolts, or wheel rim holes. It is a measurement used to determine the correct fitment of wheels on a vehicle. The PCD of a wheel is typically expressed in millimeters and is given along with the number of bolts/bolt holes. For example, a 5x100 bolt pattern means that there are five bolts evenly spaced about a 100mm bolt circle. Measuring the PCD is essential when changing wheel assemblies or rims, and it can be done by measuring from the center of one stud to the center of the stud directly opposite. The PCD measurement needs to be considered alongside the offset measurement of a wheel and hub. Getting the right PCD for a vehicle is crucial to ensure proper fitment of the wheels and safe driving.