One of the first modifications most people will do to their car is fit a nice set of alloy wheels.

The main problem when buying alloy wheels is deciding how big to go. Go too small and your alloys won’t be very noticeable, too big and the car will be slower, wheels may get damaged more easily and may rub against the wheel arches. When deciding on what size wheels to go for most cars can go 2 - 3 inches above the current stock wheel size without having any problems.

What does PCD mean?
PCD stands for pitch circle diameter and it is the diameter of an invisible circle starting from the centre of the wheel’s boltholes to the edge of the circle. PCD is measured in millimetres and also indicates the number of bolts the wheel will have. So if your car has a 4*100 PCD you will have four mounting holes or studs spaced evenly around a 100mm diameter circle.
Offset

Every car requires a unique offset. Offset is measured in millimetres of ET. The offset determines where the wheel will sit in relation to the bodyline of the vehicle. The important thing to remember is the smaller the offset the further out of your wheel arch the wheel will sit. The higher the offset the further into the wheel arch the wheel will sit.

Rolling radius
Rolling radius is the distance in millimetres from the centre of the wheel to the edge of the tire tread.
When buying larger alloys it is important to keep the correct rolling radius as it can affect your cars performance and gearing and can also cause the speedometer to be incorrect which isn’t good if you happen to be going past a speed camera and your car is going faster than what the speedometer is displaying.

If you go up a size for alloy wheels you can keep the rolling radius correct by going down a size for the profile size of the tire.
For example a 17 inch wheel with 215/55r17 can be changed to a 19 inch wheel with 215/35r19 without effecting the rolling radius.


Guide from Here