Wheel sizing

Wheels: to size a wheel, you need to know 3 things.

Bolt Pattern, and diameter of bolt circle.
Offset
Wheel size (The diameter of the wheel)

1) Bolt Pattern. For a given vehicle you'll have to know how many bolt holes there will be, for instance in a 1974 MGB, there are 4 bolt holes. You want the diameter of the 'bolt circle' in the diagram below, if you see the bolt holes(the numbered circles), the dashed line equals the 'bolt circle'. The distance is expressed in MM, or sometimes inches. When you have 4 or 6 bolt holes, it's pretty easy, you get the distance from the center of 2 opposing bolt holes. For instance in the diagram 4hole_wheel.png you would go from the center of wheel hole #1, to the center of wheel hole #4. In a 5 hole system, it's a bit more difficult, as there are not opposing bolt holes, so you must 'invent' an imaginary bolt hole that is opposing, so you can then measure the distance. For example, a 1974 MGB is 4/4.5 so it is a 4 hole bolt pattern with 4.5" bolt circle. Or in millimeters, 4/114.3.

File:INSERT 4hole wheel.png and 5hole wheel.png

2) Offset: Offset is measured in millimeters(mm). There is Positive and Negative offset. Offset is the distance from the mounting surface to the true centerline of the rim. A positive offset means the centerline of the rim/wheel is in front of the mounting surface. In other words the wheel is sunk back into the fender well more. In a negative offset it's the other direction, the wheel is stuck out of the fender, and is away from the mounting surface.

3) Wheel size: This is simply the diameter of the rim/wheel. This does not include the tire, just from one edge to the other. In a 1974 MGB, again as an example, this would be a 14" rim.