Sometimes referred to as bolt pattern or PCD: the number of stud holes on the diameter of the imaginary circle that each stud hole is centred on
Example: 5-4.75 PCD would be a 5 stud wheel and the measurement as in the diagram below for 5 stud would measure 4 3/4" or 120.65mm
5 Stud: Measure from back of the hole to the centre of the second.
4 stud: Measure from the two hole directly across from each other from center to center of the hole.
6 Stud & 8 stud: measure same as 4 stud.