Compare your current wheel with a new one and see exactly how many millimeters it moves out toward the fender and in toward the suspension, with spacers, backspacing and your own measured gaps.
| Outer lip moves | – |
|---|---|
| Inner lip moves | – |
| Track width change | – |
| Effective new offset | – |
| Backspacing, current | – |
| Backspacing, new | – |
| Clearance check | – |
Backspacing assumes overall width = nominal width + 1 inch. Static geometry only: check caliper clearance, full steering lock and suspension compression. The link in your address bar saves this setup.
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Outer change = (new width × 25.4 ÷ 2 − new ET) − (old width × 25.4 ÷ 2 − old ET). Inner change = (new width × 25.4 ÷ 2 + new ET) − (old width × 25.4 ÷ 2 + old ET). A spacer lowers effective offset by its thickness.
Example: 8″ ET40 to 9″ ET35 moves the outer lip 17.7 mm out and the inner lip 7.7 mm closer to the strut.
Offset is measured from the wheel centerline in millimeters (ET); backspacing is measured from the back lip in inches, common for trucks. Negative offset (ET−12, for example) puts the mounting face behind the centerline and pushes the wheel out.
Also compare tire diameters with the tire size calculator, confirm the hub with the bolt pattern lookup, or see the cross-section in the fitment studio.
It depends on your fender gap. Measure from the tire sidewall to the fender lip now, then subtract the outer change. Leave room for suspension compression; under about 5 mm usually rubs.
Yes. A 10 mm spacer reduces effective offset by 10 mm, moving the wheel 10 mm outward and 10 mm away from the strut.
Neither. Stay close to stock for handling and bearing load; lower offset gives a wider, more flush stance but can rub and adds scrub radius.