Two reference points, one geometry
Offset is measured from the wheel's own centreline to its mounting face. Positive offset means the mounting face is outboard of the centreline — the wheel's mounting surface sits closer to the outside face. Negative offset means the mounting face is inboard of the centreline.
Backspacing is measured from the mounting face to the inboard edge of the wheel. It is a different question: not 'where is the face relative to the wheel' but 'how far does the wheel reach inboard once it is bolted on'.
Both are correct descriptions of the same wheel. Which one you use depends on the question you are asking. If you are comparing wheel designs against each other, offset is natural. If you are asking whether a wheel will clear the suspension, backspacing is the more direct answer.
The conversion, and why width is mandatory
Because offset is measured from the centreline and backspacing from the mounting face, converting between them depends on where the mounting face sits relative to the centreline — which is exactly what offset describes — and on the total width, which is what tells you where the inboard edge is.
So the conversion requires both values. A specification that gives backspacing but omits the width cannot be converted back to an offset, and a specification quoting offset without a width cannot tell you whether the inboard lip clears anything.
The practical rule: never accept a fitment figure in isolation. A useful fitment specification states diameter, width, offset, PCD and centre bore together.
Where each one actually decides the outcome
The outboard question — does the wheel sit flush, tuck, or poke — is an offset question, because it is about where the outer face lands relative to the arch.
The inboard question — does the wheel foul the strut, the arm or the caliper — is a backspacing question, because it is about how far the wheel reaches inwards from the mounting face.
Most fitment failures we see are inboard. A wheel that looks perfect against the arch can still catch a control arm on compression, and that is not something you can assess by eye from outside the car. On a car with a large brake package, the margin is often already small with the factory wheel.
Negative offset and the brake problem
Aggressive negative offset pushes the wheel outboard, which reduces the space available on the inboard side. On a car with a big-brake option, that space is already the tightest part of the whole envelope.
The consequence is that a low-offset wheel that fits a car with standard brakes may not clear the same car optioned with the larger brake package. This is why we ask for the brake option, not just the model.
How we work it out for you
Agree the target stance first. That is a customer decision and it should be made deliberately, not arrived at by accident from a wheel that happened to be available.
We then work back to an offset that delivers the stance without consuming the clearance you need on the inboard side, and return the resulting specification — with all five values — in writing before the order is cut.
PLACEHOLDER: any specific offset or backspacing value we quote must be produced by Fenghuai engineering for the specific vehicle. Do not treat numbers from this guide as buildable specifications.