What is ET (offset)?
ET is the distance in millimetres between the wheel's mounting face and its centreline. Higher ET pulls the wheel in, lower ET pushes it out. If only ET changes, the wheel moves by the difference: going from ET45 to ET38 moves the wheel 7 mm out.
When the width changes too
Half of the width difference is added on each side (1 in = 25.4 mm):
- Inner edge = (new width − old width) ÷ 2 + (new ET − old ET)
- Outer edge = (new width − old width) ÷ 2 − (new ET − old ET)
A positive value means that edge moves towards its own side: the inner edge towards the strut, the outer edge towards the arch.
How much is acceptable?
It depends on each car's clearances. Staying within ±5 mm of the factory ET is fine on most cars. An outer edge more than 10 mm further out needs a careful check. The bolt pattern (PCD) and centre bore must match the car as well.