The PCD defines the spacing of the bolts that attach the wheel to the hub of your car. You can get 3 stud, 4 stud, 5 stud and even 6 stud alloy wheels. The more powerful the car the more studs the wheels tend to have. Just because you have a car with a 4 stud hub doesn’t mean that any 4 stud alloy will fit. It’s never that easy. Different manufacturers use different PCD’s.

The diagram below shows how a circle can be drawn with its circumference passing through the centre of every bolt. The diameter of this circle is the Pitch Circle Diameter. As you can see it’s possible to have two 4 stud alloys with the bolts spaced differently to give a different PCD.

So how do you measure the PCD for your car? An example would be measurement (A) below is 100mm resulting in a Stud Pattern or P.C.D of 4x100 or measurement (B) below is 108mm resulting in a Stud Pattern or P.C.D of 4x108.

If the below were 5 stud the Stud Pattern or P.C.D would be 5x100 and 5x108

The Offset (ET) dimension of your wheel is the distance between the centerline of your rim and the surface where the alloy contacts the hub. If the offset is incorrect your wheel may protrude from the wheel arch (which can look cool, but is illegal) or on the other hand it may sit deep in the arch and look crap. Combine this with the wrong rim width and you could end up with a wheel that rubs the inside of your arch and reduces your turning circle.

Even if you buy new wheels that have the same PCD and offset as your old ones, it doesn’t necessarily mean that they will fit. The centre bore of the wheel and hub must match to ensure correct centralisation of the wheel and the spokes design has to allow enough clearance for your brake calipers.