Pitch Circle Diameter
- A rim's pitch circle diameter (PCD) value is the diameter of an imaginary circle drawn using the wheel mounting holes as a guide. This is measured in millimeters (mm) and will be listed in conjunction with the number of mounting holes, e.g. 4x100, meaning four mounting holes with a PCD of 100 mm.
- When fitting rims, the spigot is the protruding piece from the center axle on which the wheel is mounted. The spigot size must match that of the rim, or an adapter will be needed. This can take the form of an aluminum ring placed between the rim and the spigot.
Diameter and Depth
- A wheel's diameter and depth are used to make sure the wheel fits within the car's wheel well. When up-sizing rims from 14 inches to 16 inches, a lower-profile tire will often need to be used to provide enough clearance for the wheel well. The tire must match the wheel's diameter and depth values to fit properly.
- Wheel offset, measured in millimeters, is the distance between the rim's central plane and the plate on which it is mounted. Offset is expressed as a positive or negative number. Positive offset means the mounting point is toward the road side from the central plane. Negative offset means the rim's central plane is toward the hub side from the mounting point. Each unique car model has a proper wheel offset range value range which absolutely must be followed to avoid suspension damage.