What is a Center Bore? Hub Centric vs. Lug Centric
The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types:
The hub mounting surface is even with the centerline of the wheel.
The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.
The hub mounting surface is toward the back or brake side of the wheels centerline.
"Deep dish" wheels typically have negative offset or a very low positive offset.
Calculating the Offset of a Wheel
Calculating the offset of a wheel is a fairly easy mathematical equation. First, measure the overall width of the wheel (remember, just because a wheel is 18x7.5, does not mean that the OVERALL width is 7.5". It means that the measurement from outboard flange to the inboard flange is 7.5"). Next, divide that width of the wheel by two; this will give you the centerline of the wheel.
Overall width/2 = Centerline
After determining the centerline, measure from the mounting pad to the edge of the inboard flange (if the wheel were laying flat on the ground - face up - your measurement would be from the ground to the mounting pad). This is your back spacing.
Centerline - Back Spacing = Offset in Inches
Inches x 25.4 = Offset in mm
All of the wheels in the American Racing catalog indicate the wheel's offset.
Determining the Right Offset for Your Car
Short of adding fender flares or a body kit to your car or rolling and pulling your fenderwells, there is no way to significantly change your car's offset. If your car has a high offset, you will have to buy a high offset aftermarket wheel. If you car has a low offset, you will need a low offset aftermarket wheel. Typically, front wheel drive vehicles have a high offset (+35 mm or greater), and rear-wheel drive applications will have lower offsets (this is not true in all cases, as the Honda S2000 is rear wheel drive, and has a very high offset). Your installer or tire and wheel dealer should be able to tell you what the offset of your vehicle is.
The common definitions are:
Other factors in determining what offset is right for your vehicle is whether not you are making any other changes to your car (lowering or raising, aftermarket brakes, etc.).
If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. For most cars, this won't work correctly.