ET40 vs ET30
Going from ET40 to ET30 moves each wheel 10 mm outward. Track width changes by +20 mm across the axle.
A 10 mm change is noticeable. Check the arch lip on full lock and compression before committing.
What changes
| Measurement | Change |
|---|---|
| Wheel position | 10 mm outward |
| Outer clearance (to the arch) | -10 mm |
| Inner clearance (to strut and spring) | +10 mm |
| Track width (both wheels) | +20 mm |
| Offset change | -10 mm |
Offset is a one-for-one measurement: every millimetre removed from the ET number is a millimetre further out, and a millimetre gained on the inside. This holds at any wheel width, because offset moves the whole wheel rather than changing its size.
Where the rim edges sit
Wheel width does change where the edges land, because extra width splits either side of the mounting face. Distances below are measured from the hub face.
| Width | Outer at ET40 | Outer at ET30 | Inner at ET40 | Inner at ET30 |
|---|---|---|---|---|
| 6.5J | 42.5 mm | 52.5 mm | 122.5 mm | 112.5 mm |
| 7J | 48.9 mm | 58.9 mm | 128.9 mm | 118.9 mm |
| 7.5J | 55.3 mm | 65.3 mm | 135.3 mm | 125.3 mm |
| 8J | 61.6 mm | 71.6 mm | 141.6 mm | 131.6 mm |
| 8.5J | 67.9 mm | 77.9 mm | 147.9 mm | 137.9 mm |
| 9J | 74.3 mm | 84.3 mm | 154.3 mm | 144.3 mm |
| 9.5J | 80.6 mm | 90.6 mm | 160.6 mm | 150.6 mm |
| 10J | 87.0 mm | 97.0 mm | 167.0 mm | 157.0 mm |
Moving outward
Lower offset pushes the wheel toward the arch. The gain is inner clearance — useful if a wider wheel is fouling the strut — and the cost is arch clearance, which is where fouling shows up first, usually on compression with the steering turned. It also widens track, which slightly increases scrub radius and can make the steering feel heavier and more sensitive to road camber.
These figures are the geometry of the offset change alone. Your car's arch shape, liner position, suspension travel and ride height all decide whether the clearance you have is enough. Measure before you buy.
Model it with your wheel and tyre
The fitment calculator takes wheel width and tyre size alongside offset, so you can see the combined effect rather than offset in isolation.
Related offset changes
How are these fitment figures calculated?
Fitment North calculates declared wheel and tyre geometry from the dimensions shown on the page, using the same tested method as the wheel-offset and tyre-size calculator. Tyre diameter uses rim diameter plus two calculated sidewalls; offset comparisons split width changes across inner and outer edges before applying the offset difference. Results are measurement aids, not vehicle approval. Confirm ratings, clearances and manufacturer guidance before fitting parts.