How it is calculated
Rim width is in inches and ET is in millimetres. The rubber, any spacers, and whether the wheel meets the caliper are not checked.
Outward = (new width − old width) / 2 × 25,4 − (new ET − old ET)
Where the numbers come from
- Old width — unit in the field: in
- Old ET — unit in the field: mm
- New width — unit in the field: in
- New ET — unit in the field: mm
Example
It was 7 inches ET 40, now 8 inches ET 35. About 17,7 mm outward.
Questions
A negative number?
The edge moved inward relative to the old wheel.
A smaller ET means the wheel sits outward?
Yes. A smaller offset moves the mounting face out, and at the same width the wheel sticks out further.
Is this permission to install it?
No. You also need the centre bore, the bolt pattern, and an offset the hub will accept.