Pitch Circle Diameter
PCD stands for Pitch Circle Diameter. It is the diameter of a circle that is used to set the wheel nuts on.
When there are 4 or 6 studs, measuring the PCD is easy. Just measure between opposing nuts, taking the measurements from the center of the nut to the opposite center.
To calculate the P.C.D. of a five stud wheel or hub, measure from the inside of one hole to the outside of the next hole and multiply by 1.7012. The picture below shows where to measure from