Guide
What wheel offset do I need
Offset alone does not say where the wheel ends up. Width and offset together do.
Short answer
Start from your stock wheel's width and offset, which are stamped inside the wheel, and keep the new wheel's inner edge no further in and its outer edge no further out than you have room for. Going from an 8 in wheel at +45 mm to a 9 in wheel at +35 mm moves the outer edge 23 mm further out and the inner edge 3 mm further in.
What offset means
Offset is the distance in millimetres from the centreline of the wheel to the face that bolts to the hub. Positive offset puts that face toward the outside of the wheel, so the wheel tucks in under the car; most modern cars are +35 to +50. Zero puts it in the middle. Negative offset puts it toward the inside, so the wheel pokes out, which is common on lifted trucks. Lower offset always means the wheel sits further out.
Width and offset together
A wider wheel grows by half its extra width on each side. Changing the offset then shifts the whole wheel in or out. So the outer edge moves out by half the extra width plus the drop in offset, and the inner edge moves in by half the extra width minus the drop in offset:
| Wheel | Inner edge | Outer edge | Backspacing |
|---|---|---|---|
| 8 in, +45 mm | 147 | 57 | 6.27 in |
| 8 in, +35 mm | 137 | 67 | 5.88 in |
| 9 in, +35 mm | 149 | 79 | 6.38 in |
| 9 in, +0 mm | 114 | 114 | 5.00 in |
| 9 in, −12 mm | 102 | 126 | 4.53 in |
| 10 in, −24 mm | 103 | 151 | 4.56 in |
Inner and outer edge are measured at the bead seat; the flanges add roughly half an inch each side, which is why backspacing, measured to the lip, is a little more than the inner edge.
How to pick an offset
- Find the stock width and offset: the offset is stamped inside the wheel as ET or a figure in mm.
- Measure the room you have: from the inner lip to the nearest suspension part, and from the outer lip to the fender edge, with the wheel pointing straight ahead.
- Choose a width, then an offset that keeps the inner edge inside the first gap and the outer edge inside the second. Leave a few millimetres for suspension travel and steering lock.
On most cars, staying within about 5 to 10 mm of the stock offset for the same width keeps everything the factory tested. Large changes move the scrub radius, which changes steering feel and puts more load on the wheel bearings.