How to use this calculator
- Enter the current wheel. Enter current wheel width and offset.
- Enter the new wheel. Enter the proposed wheel width and offset.
- Add spacer thickness. Enter spacer thickness if used; otherwise leave zero.
- Read clearance and poke. Positive inner clearance means more room at the suspension; positive poke means the outer lip moves outward.
How it works
Wheel offset is measured from the wheel centerline to the hub mounting face. Positive offset increases inner backspacing; a spacer reduces effective offset. The calculator converts wheel width from inches to millimetres, then compares inner and outer positions from backspacing = width/2 + offset.
Inner clearance change is current backspacing minus new backspacing. Positive means the inner lip moved away from the strut or suspension. Outer poke change compares the outside lip, so positive means the wheel moves outward. Pair this with the tire size calculator because sidewall width and diameter can matter more than the wheel lip alone.
Worked example
Verified against the live calculator
Moving from an 8 in ET35 wheel to a 9 in ET30 wheel adds 25.4 mm of width and 5 mm less offset. The inner lip moves about 7.7 mm closer to the suspension and the outer lip moves about 17.7 mm outward.
Frequently asked questions
What does positive wheel offset mean?
Positive offset means the mounting pad is outward from the wheel centerline. It increases backspacing and usually moves the inner lip closer to the suspension.
How does a spacer affect offset?
A spacer moves the wheel outward, so effective offset is wheel offset minus spacer thickness.
What is backspacing?
Backspacing is the distance from the mounting face to the inner wheel lip. This calculator estimates it from nominal wheel width and offset.
Does this prove a wheel will fit?
No. It is a geometry screen. Tire sidewall shape, fender clearance, brake caliper clearance, suspension travel and alignment still need inspection.
Method & assumptions
- Uses nominal wheel width. Real wheel flange width varies by manufacturer.
- Spacer thickness is subtracted from the new wheel offset.
- Does not check caliper barrel clearance, tire bulge, fender clearance, scrub radius, suspension compression or steering lock.