Since there has been a LOT of questions about it (including myself).. i thought maybe we should have a sticky for it defining what it is, and how its calculated.. please add / amend this so eveyone can understand it..
OFFSETOffset (ET)
The offset of a wheel is the distance in millimetres between the mounting face and the centre line of the wheel.
Positive offset wheels have the mounting face closer to the outside of wheel, forward of the centre line. This will place the wheel further under the car. These wheels generally have very little dish on the outer rim and a small wheel boss on the back of the wheel. Eg. Subaru & Honda
Negative offset wheels have the mounting face towards the back of the wheel, rear of the centre line. This results in the wheel sitting further out from the car. These wheels can have a deep dish on the outer rim and/or a large wheel boss on the back. Eg. Pre AU & 4WD wheels.
Calculating Offsets
* To find the offset of a wheel, first measure the backspace of the wheel from bolt up face to back edge of wheel
* Measure the overall physical rim width (edge to edge) or add 1 inch to the manufacturer’s stamped wheel size.
* Multiply by 25.4 to convert to millimetres and then divide by 2 to find the centre line of the wheel.
* Subtract the centre line measurement from the backspace measurement.
* This will give you either a positive or negative offset measurement.
In most cases, changing the offset of a wheel will increase the loads on bearings, axles, suspension joints and steering components. For this reason it is important to fit wheels with a correct offset to prevent premature wear or suspension failure.
WHEEL DIMENSIONSWheel Width
Width of the wheel measured from outer bead seat to inner bead seat. This is not the same as the physical width of the wheel.
Wheel Diameter
Diameter of the wheel measured from the bead seat to opposite bead seat. The physical size of the wheel will be a larger diameter. Centre line Half of rim width measured from edge to edge.
Rear Spacing
The distance from the mounting face to the outside edge of the back of the wheel. This can be measured by placing a straight edge across the back of the wheel and measuring to the mount face. This distance can be used to calculate offsets.