Changing your vehicle's OEM (Original Equipment Manufactured) wheels to more desirable aftermarket ones is always an exciting idea, but there are things to keep in mind to get that right wheel fitment. One of the important things to look out for is the wheel offset. A wheel offset relates to the distance between the wheel's mounting hub and centerline. If the face of the mounting hub sits behind the centerline (so the mounting hub sits "inside" the barrel of the wheel), that means the wheel has a negative offset. Whereas if it sits ahead of the centerline (meaning the mounting hub is further out towards the edge of the car), it has a positive offset (see Illustration). Offset is measured in 'ET' (Einpresstiefe, a German word used to describe depth of insertion). The higher the "ET number," the more positive the offset is from the wheel's centerline, while a negative number translates to negative offset.
Why is wheel offset important? Changing offset not only affects the look of a wheel and the car, which is what most of us are so concerned about. But it also affects the way the suspension reacts to movements from the wheel. Since the suspension assembly is attached to the wheel's mounting hub, the angle and forces transmitted from the wheel to the suspension components are slightly different. This in turn could affect the car's steering response and characteristics if the wheel offset has been changed significantly. Extreme offsets will leave the car scrambling for grip in turns as the Tire and the Suspension are lined up together wheel.
Some wheels will also not fit on a particular car as that wheel's offset pushes the wheel into the suspension, or mounts it too close to clear the bodywork. There are plenty of calculators online to help work out what offset your car needs like www.1010tires.com/Tools/Wheel-Offset-Calculator, and any reputable Car Accessory Shop will be able to advise what offset your car will need, when changing wheel or tire size.
How to Measure Offset
and Back Spacing Clearance
Knowing your vehicle's backspacing clearance is essential in finding the right wheel offset. Your wheel's backspacing is measured from the mounting surface in the middle to the back edge of the wheel. It is best to measure the backspacing of your OEM wheel and the clearances from the suspension to see your options when choosing an aftermarket wheel.
Aftermarket wheels come in so many offsets from Oldschool – 13's to the widely used +32 to +50. If it has zero offset, the hub mounting surface is even with the wheel's center line. If it has positive offset, the mounting surface is closer to the front of the wheel, while negative offset is closer to the back of the wheel.
Offset is measured in millimeters(MM). If you purchase wheels with the wrong offset, it's likely it won't look good or worse, it won't fit.