Wheel size is a critical factor which should affect your decision when choosing which wheel is right for you. So its critical that you know the wheel size variables.
There are 4 wheel size variables, yes four.. and you need to know what they are. They are 1. bolt pattern, 2. diameter, 3. width and 4. offset. This is how they are laid out:
The bolt pattern is comprised of two numbers – the first indicates how many bolt holes are on the wheel, and the other describes how far apart they are. A bolt pattern of 5-120 would mean the wheel has five bolt holes and they are 120mm apart.
Therefore, depending on your car, the bolt pattern is different. E.g. most Japanese cars have a bolt pattern of 5×114.3, and most of German cars have a bolt pattern of 5×120.
This is the easiest one which most people know. Its size of the wheel as measured across its center (measured in inches).
The width of the wheel as measured across its barrel from the front rim to the rear rim (measured in inches).
The offset is the tricky part where most people don’t get, so concentrate here. The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel (measured in millimeters). Look at the diagram to understand it more.
A car can support many different variations of wheel sizes (except bolt pattern), depending on your needs and usage, you might want to choose large wheels, wide wheels, have aggressive or conservative offsets. However, choosing the wrong size may result in your wheel not clearing your calipers, not clearing your suspension or rub your fenders, also wrong wheel size can affect the handling of your car negatively. So, before you jump on buying wheels, do your research on whats the best size for you. As stated many times before, looking on forums is a very good way to do so.