How this calculator works
Wheel offset is the distance from the wheel mounting face to the centreline of the wheel. Positive offset means the mounting face is outboard of the centreline, which tucks the wheel inboard; negative offset pushes it outward. Backspace is the distance from the mounting face to the inner lip, so it equals half the width plus the offset. Comparing the backspace of two wheels shows how much closer the new inner edge sits to the suspension, while the change in the outer edge shows how much farther the wheel may protrude. Width and offset changes combine, so a wider wheel with a lower offset can move the inner edge very little while pushing the outer edge out a long way.
Formula
Backspace = width / 2 + offset. Inner edge movement = new backspace - old backspace, positive toward the suspension. Outer edge movement = (new width - new backspace) - (old width - old backspace), positive outward.
Worked example
An 8 inch wheel with +45 mm offset has a backspace of 101.6 + 45 = 146.6 mm. A 9 inch wheel with +35 mm offset has 114.3 + 35 = 149.3 mm, so the inner edge moves 2.7 mm toward the suspension while the outer edge moves 22.7 mm outward.
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Frequently asked questions
Does a lower offset always push the wheel out?
At the same width, yes. Lowering the offset moves the mounting face toward the centreline, reducing backspace so the wheel sits farther out. A wider wheel can offset that effect on the inner side while still adding width on the outer side.
How much clearance do I need?
There is no universal number. Check the gap to suspension components and the fender on both sides at full lock and full suspension travel. Tire section width and sidewall shape can be wider than the wheel itself.
Why is backspace used instead of offset?
Backspace tells you directly how deep the wheel sits under the mounting face, which is what matters for inner clearance. Offset is easier to compare between widths because it is measured from the centreline.
Can I use this with staggered wheels?
Yes. Enter one axle at a time, for example the original front and the new front, then repeat for the rear. The page compares any two wheels, not a full set.