How to find out what size Rings are needed?

To find out what size rings are needed you will have to find out the center bore of the vehicles hub.  This can be got at www.wheelfitment.eu or by taking off a wheel and measuring the hub.   To get the measurement of the wheel, you just need to put the wheel on its face and measure the center bore at the rear.

The rings listed below are commonly found on BMW and Opel models with a 72.6mm hub.

Please note that if the center bore is smaller than 72.6mm, i.e. 57.1 – 72.6mm, then these rings can only be used when fitting wheels from the makes listed above onto vehicles with a smaller bore.

Why are Spigot Rings needed?

The rings are needed when the hub hole (the center bore) on the wheel is larger than the wheel hub of the vehicle.  They are usually required for aftermarket wheels since the original wheels come with a center bore of the right size.  They can however be used on original wheels when fitting those wheels onto a different make or model which has a smaller hub.
Usually, the hub-centric rings are used with alloy wheels, but sometimes steel wheels may need them.
The purpose of the centric ring is to perfectly center the alloy wheel to the wheel hub and prevent vibration to the steering wheel during driving and shaking which typically occurs between speeds of 65-95 km/h.