Pitch Circle Diameter, more commonly known by the abbreviation PCD, is the measurement of the diameter of an imaginary circle drawn through all of the wheel bolts or studs on a wheel.
4-Stud Hubs - Measuring the PCD of 4 or 6-stud wheel hubs is straightforward, just measure the distance from the outside of one bolt hole to the inside of the opposite hole as shown in the image below.
5-Stud Hubs - This is a bit trickier, but still not difficult. Measure the distance from the outside of any bolt hole to the inside of the adjacent hole as shown in the image below - now multiply that figure by 1.7012 e.g if the measurement of 'X' is 65.8mm then the PCD will be 112mm (65.8 x 1.7012 - rounded to the nearest decimal point)
Thus, a wheel spacer advertised as 5x112 PCD will fit cars that use 5 wheel bolts per hub and that have a pitch circle diameter of 112mm