# How will a new wheel offset fit?

Compares a new wheel with the old one from their widths and offsets: how far the inside edge moves toward the suspension and the outside edge toward the fender, and the change in track width.

- Page: https://www.acalculator.org/everyday-life/wheel-offset-calculator
- JSON spec: https://www.acalculator.org/everyday-life/wheel-offset-calculator.json
- Version: 2d90860d804d

## Default answer

Example with the default inputs (Current wheel width 8 in, Current wheel offset 35 mm, New wheel width 9 in, New wheel offset 30 mm): The new wheel’s outside edge moves out by 17.7 mm and its inside edge moves in by 7.7 mm.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| w1 | Current wheel width | The rim width of the wheel on the car now, usually in inches. |
| et1 | Current wheel offset | The offset (ET) of the wheel on the car now, usually in mm. |
| w2 | New wheel width | The rim width of the new wheel, usually in inches. |
| et2 | New wheel offset | The offset (ET) of the new wheel, usually in mm. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| outward | Outside edge moves out | How much further out, toward the fender, the new wheel’s outside edge sits; negative means it sits further in. |
| inward | Inside edge moves in | How much further in, toward the suspension, the new wheel’s inside edge sits; negative means more room. |
| track | Track width change | The change in the distance between the outside edges of two wheels on one axle: twice the outside change. |
| offsetChange | Offset change | New offset − current offset. |

## Method

outside change = (new width − current width) ÷ 2 − (new offset − current offset); inside change = (new width − current width) ÷ 2 + (new offset − current offset)

## Assumptions

- Widths are rim widths (the “9” in 18×9), measured the same way for both wheels, usually in inches (1 in = 25.4 mm); either field takes inches or millimeters.
- Offset (ET) is the distance from the wheel’s centerline to its hub mounting surface, positive toward the outside face.
- The tire is not included: a wider tire on the same wheel sticks out further on both sides.

## Worked examples

1. w1 = 0.2032, et1 = 0.035, w2 = 0.2286, et2 = 0.03 gives outward = 0.0177, inward = 0.0077, track = 0.0354, offsetChange = -0.005. Source: Tire Rack, What Is Wheel Offset? (offset = distance from the hub mounting surface to the wheel’s centerline; https://www.tirerack.com/upgrade-garage/what-is-wheel-offset).
2. w1 = 0.1778, et1 = 0.045, w2 = 0.1778, et2 = 0.02 gives outward = 0.025, inward = -0.025, track = 0.05.
3. w1 = 0.2159, et1 = 0.04, w2 = 0.2413, et2 = 0.05 gives outward = 0.0027, inward = 0.0227, track = 0.0054, offsetChange = 0.01. Source: Half an inch is 12.7 mm (NIST SP 811 (2008), Appendix B: 1 in = 25.4 mm).

## FAQ

### What is wheel offset?

Offset (the ET number stamped on the wheel) is the distance in millimeters from the wheel’s centerline to the surface that bolts to the hub. A positive offset puts the mounting surface toward the outside face, which tucks the wheel in; a negative offset pushes it out.

### How does a lower offset change the fit?

Every millimeter less offset moves the whole wheel 1 mm out: the outside edge 1 mm closer to the fender and the inside edge 1 mm further from the suspension. Going from ET45 to ET20 on the same width moves the wheel 25 mm out.

### How does a wider wheel change the fit?

A wider wheel grows on both sides of its centerline, half the extra width each way. One inch wider at the same offset sticks out 12.7 mm further and reaches 12.7 mm further in.

### How do I keep the outside edge in the same place with a wider wheel?

Raise the offset by half the extra width. One inch wider needs 12.7 mm more offset; all the extra width then goes inward, so check the room at the suspension.

### What is backspacing?

Backspacing is the distance from the hub mounting surface to the inside lip of the wheel, usually in inches. It depends on the offset and the wheel’s overall width including the lips, so measure it on the wheel.

### Does this include the tire?

No. The page compares the wheels only. A wider tire on the same wheel sticks out further on both sides, and a taller tire changes the room to the fender and suspension too.

## Sources

- Tire Rack, What Is Wheel Offset?: offset is the distance from the hub mounting surface to the wheel’s centerline; positive offset tucks the wheel in. https://www.tirerack.com/upgrade-garage/what-is-wheel-offset
- NIST SP 811 (2008), Appendix B: 1 inch = 25.4 mm exactly.