Wheel sizing

In order to size a wheel for an automobile, an owner needs to know a few fundamental things:

• The bolt pattern
• The diameter of the bolt circle
• The offset
• The size of the wheel
• The centerbore
  

  Bolt pattern

Bolt circle

The bolt circle is the circle determined by the positions of the bolts; the center of every bolt lies on the circumference of the bolt circle.

The important measurement is the diameter of the bolt circle, usually expressed in millimeters, although inches are sometimes used. For a 4- or 6-bolt car, this measurement is merely the distance between the center of two diametrically opposite bolts. In the 4-bolt picture below, this would be the distance between holes #1 and #4. Some basic geometry is needed to find the center of a 5-bolt pattern: draw a line between any two neighboring bolts, and draw a line from this line to the opposite bolt. Repeat with a different set of three bolts, and the two long lines will cross in the center, thereby making the distance between this intersection and a bolt the radius of the bolt circle.

A 1974 MG B is a 4/4.5" (4/114.3) car, meaning it is, again, a 4-bolt pattern with a 4.5" or 114.3mm bolt circle.

Lug nuts or bolts

Another thing to consider when new wheels are purchased is proper lug nuts or bolts. They are usually either flat, tapered, or ball seats, meaning the mounting surfaces are flat, tapered, or spherical respictively. For expample, most Hondas have ball lug seats from the factory while most aftermarket wheels have a tapered lug design. If you buy aftermarket wheels for a Honda make sure you get the proper lug nuts for the wheel or the wheel will not be properly centered.
Some aftermarket wheels will only fit smaller lug nuts, or not allow an ordinary lug nut to be properly torqued down because a socket will not fit into the lug hole. Tuner lug nuts were created to solve the problem. Tuner lug nuts utilize a special key to allow removal and isntallation with standard lug wrench or socket. The design of tuner lug nuts can range from spline drive to multisided, and are sometimes lightweight for performance purposes.

• Make sure to keep a set of lugs that fit your spare tire.
• Keep a key to aftermarket lug nuts in the car in case of a flat or worse
  

  Offset

Wheel size

The wheel size is the diameter of the wheel, in inches, not counting the tire. A 1974 MG B has a 14" rim, for example.

Centerbore

The centerbore of a wheel is the size of the hole in the back of the wheel that centers it over the mounting hub of the car. Factory wheels have a centerbore that matches exactly with the hub to reduce vibration by keeping the wheel centered. Wheels with the correct centerbore to the car they will be mounted on are known as hubcentric.
Hubcentric wheels take the stress off the lug nuts, reducing the job of the lug nuts to holding the wheel to the car. Wheels that are not hubcentric are known as lugcentric, as the job of centering is done by the lug nuts assuming they are properly torqued down.
Centerbore on aftermarket wheels must be greater than or equal to that of the car or the wheel will not physically mount to the car. Many aftermarket wheels come with hubcentric rings that lock into the back of the wheel to center it like a factory wheel, usually made of plastic.