Wheel Fitment Guide

From coded markings to technical descriptions, this simple guide will give you a better understanding of the wheel and the jargons which come along with it. In order to size a wheel for an automobile, you need to know a few terms:

• Wheel Size
• Bolt Pattern (P.C.D.)
• Offset
• Centerbore

Wheel Size (rim marking)
Look inside almost any wheel and you should find various markings which give you pertinent information on the wheel. Just for conversation sake, we will take into consideration a wheel which is marked "18x8J ET38". The first number is quite obvious and represents that the wheel is 18 inches in diameter, while "8" is the width of the wheel and measured from the distance between the flanges that support the bead. The "J" refers to the shape of that flange. Easier to understand if you imagine a steel wheel on which the lip is rolled over like a J. "ET" is the abbreviation of einpress tief, German that translates literally as "pushed in depth", or offset. The "38" is the offset measurement "38 mm".

Bolt Pattern: (P.C.D)
The P.C.D. (Pitch Circle Diameter), often referred to as the "bolt pattern", refers to the pitch circle diameter of the bolt holes. It is the diameter of an imaginary circle formed by the centers of the wheel lugs/bolts. The number of bolts depending on the vehicle type, usually four, five or six for passenger cars. Some aluminum wheels available on the market are multi-hole wheels, such as the nine-hole type that has four plus five bolt holes. A bolt circle of 4x100 would indicate a four –lug pattern on a circle with a diameter of 100mm.

Bolt Pattern Conversion chart

Inches	Millimeters
5 x 4	5 x 100
5 x 4.25	5 x 107.95
5 x 4.5	5 x 114.3
5 x 4.72	5 x 120
5 x 5.12	5 x 130

Offset
Offset is the location of the flat mounting surface of a wheel relative to the wheel’s centerline. Negative offset means that the mounting surface is toward the center of the car. Positive offset means that it’s toward the outside of the car, or the wheel is “pulled in” toward the center. While not directly a matter of offset, brake caliper clearance is a related issue. If you plan to have big brakes on your car, be sure that your wheels will fit over the calipers. Spacers are available to solve big brake kit fitment issues. But it’s best to get a wheel with enough clearance to meet your offset specs and still fit your brakes. Many after-market brake companies even have templates of their brakes available that you can easily check against any wheel.

Centerbore
The other element that affects directly whether a wheel can be bolted onto a car is hubcentricity. The centerbore of a wheel is the size of the hole in the back of the wheel that centers over the mounting hub of the car. Factory wheels are all machined to fit their specific application exactly, and some of the better after-market wheels are, too. However, many after-market wheels rely on hub centric rings. This means that instead of machining wheels specifically for each O.E. centering hole diameter, the wheel manufacturer machines all wheels to one size, and then uses inserts to give a centering surface of the diameter required for each application.

Tire Sizing
To help you choose tire sizes we created this Tire Size Calculator.
1. Input your stock and new tire sizes in the first and second row respectively
2. Click "Calculate" to see how the new tire compares to the stock tire.
3. Adjust the parameters of the new tire to closely match the stock tire's diameter.
(The graphics shows the stock tire in black outline, and new tire in solid blue.)