What is alloy wheel backspacing? What is the ET number on alloy wheels? What is negative offset? How do you measure wheel offset?”

Alloy wheel offset is one of the measurements that makes up its wheel fitment along with PCD, diameter & width, offset is one of the core measurements you’ll need to make sure a wheel will fit onto your car of choice.

Offset is the distance in millimetres from the centre line of the wheel to the wheel’s mounting face. Given that the mounting face can be either in front of or behind the centreline, the offset can be either neutral, positive or negative. (see image)

Zero or neutral offset is when the wheel’s mounting face is exactly in line with the centre line of the wheel. This means that they both line up and that there is no difference between the wheel itself and the wheel arches – zero offset wheels are often referred to as wheels with ET0

ET0 Zero or neutral offset https://mywheelsandtyres.co.nz/product/talon/

Positive offset is when the mounting face is in front of the centre line of the wheel. Looking at the wheel straight-on from the front, wheels with positive offsets tend to have a flat style or very occasionally a small dish or a slightly concave shape.

ET30/35 Positive offset https://mywheelsandtyres.co.nz/product/bond/

Negative offset have the mounting face located behind the centre line of the wheel. This means that the mounting face of the wheel sits much further into it. Viewed from the front, these wheels often sport very aggressive fits with lots of concave or extreme dishes.

ET-12 Negative offset https://mywheelsandtyres.co.nz/product/hummer-4/


Remember those two little letters from up above? ET stands for einpresstiefe, or for those whose German isn’t up to scratch, insertion depth. This is a number stamped on the rear spokes or mounting face of an alloy wheel. The ET of a wheel is the measurement in mm of how far the wheel’s centre line is from its mounting face.

ET wheel numbers can be either positive or negative to reflect the values of wheels with either positive offsets or negative offsets. For example, an ET45 wheel measurement has a positive offset of 45mm, which means that the mounting face is 45mm in front of the centre line. Conversely, a wheel with ET-12 will have a negative offset where the mounting face is 12mm behind the centre line of the wheel.


While wheel offset is the distance from the centre of the wheel to the mounting face, wheel backspacing is the measurement of distance from the back edge of the wheel rim to the mounting face. In other words, the space in the back of the wheel.

Essentially, wheel backspacing is a combination of the wheel offset measurement and width. It is important if you’re putting new alloy wheels onto your vehicles that are physically wider than the wheels you had on it before. In this case, you may need to change offset in order to compensate for the greater width.