Wheel Offset Calculator

Original rim width, inches

Original offset ET, mm

New rim width, inches

New offset ET, mm

Result

VerdictThe wheel moves inboard: check the clearance to the strut
Shift outward, mm17.7
Shift inward, mm7.7
ET for the same outer position57.7
Spacer requiredno spacer needed
Change in ET, mm-5

Width and offset work together

Shows how far a new wheel moves outward towards the arch and inward towards the strut compared with the original, and what spacer would cancel the difference.

Offset, marked ET, is the distance from the mounting face to the centreline of the wheel. A larger ET tucks the wheel deeper into the arch. On its own the number says nothing: whether the wheel moves depends on ET and width at the same time.

The arithmetic is simple: half the rim width minus the offset gives the distance from the hub face to the outer edge. That is why a wider rim with a smaller ET moves outward twice over — once for the width, once for the offset.

What this calculator shows

The outward shift, towards the arch lip and the wing
The inward shift, towards the strut, the caliper and the liner
The offset a wider rim would need to sit exactly where the original did

What to keep in mind

The calculation is geometric: it knows nothing about spoke shape, caliper clearance or the real dimensions of your arches.
Any shift over 5 mm is worth checking on a lifted wheel at full lock and with the suspension compressed.

FAQs

What is wheel offset in plain terms?

The distance in millimetres from the face that bolts to the hub to the middle of the rim. A positive offset means the mounting face is outboard of the centreline, so the wheel sits deeper in the arch.

Does a lower ET always stick out more?

At the same width, yes. Dropping ET by 10 mm moves the wheel exactly 10 mm outward and takes the same 10 mm away from the strut.

Can spacers fix the offset?

Outward, yes — a spacer moves the wheel by exactly its own thickness. Nothing moves a wheel inward, so a rim with too little offset cannot be corrected.

What goes wrong with the wrong offset?

Too little and the tyre rubs the arch and liner while the hub bearing takes extra load. Too much and the rim fouls the strut or caliper. Check both on a lifted wheel at full lock.

Worked example

From 6.5J ET45 to 7.5J ET40

Input: 6.5J ET45 → 7.5J ET40

Output: 17.7 mm further out, 7.7 mm further in

Note: Nearly two centimetres outward is the level at which a loaded car starts catching the arch lip. To sit where the original did on the outside, the wider rim would need ET 57.7 — and wheels like that barely exist.

Useful links

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Wheel Offset Calculator

Result

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