Also called rear spacing. It is the distance from the extreme back edge of the rim, not bead seat, to the mounting pad, back of where the wheel mounts to the hub. To calculate, lay a straight edge across the back edge of the rim and measure down to the mounting pad. (Refer to Diagram on Above)
Wheel Offset is the distance from the wheels centerline to the plane at which the wheel mounts to the hub. Positive offset is towards the outside of the rim, Negative offset is towards the inside. Zero offset is at the centre of the rim.
PCD - Pitch Circle Diameter
PCD (Pitch Circle Diametre) is the diametre of a circle drawn through the centre of your wheels bolt holes. The PCD is measured in either inches or millimetres and also indicates the number of studs or bolts the wheel will have.
To convert inches to millimetres simply multiply by 25.4. e.g. 6/5.5 = 6/139.7 (5.5 X 25.4 = 139.7)