Wheel offset explained

Offset is the single most misunderstood wheel spec, but the idea is simple: it decides where the wheel sits in the arch relative to its mounting face. Get it wrong and the wheel either tucks in and rubs the strut, or pokes out past the fender.

What offset (ET) actually is

Offset is the distance from the wheel's mounting face (where it bolts to the hub) to its centreline. It is stamped on the wheel as ET (from the German Einpresstiefe) in millimetres.

• Positive offset (e.g. ET45): the mounting face is outboard of the centreline, pulling the wheel inward toward the suspension. Typical for front-wheel-drive and most modern cars.
• Zero offset (ET0): the mounting face is exactly on the centreline.
• Negative offset (e.g. ET-12): the mounting face is inboard, pushing the wheel outward — the classic deep-dish or "poke" look.

Backspacing — the same idea, different units

Backspacing is the distance from the mounting face to the inner lip of the wheel. It describes the same geometry as offset but is measured in inches from the back. The two are linked through the wheel width:

backspacing = (width ÷ 2) + (offset ÷ 25.4) + 0.5"

The half-inch accounts for the two lips. In practice, backspacing tells you how much room there is on the inboard side for the strut, brakes and suspension, while offset tells you where the whole wheel sits.

How offset changes fitment

Lowering the offset number (more negative) by, say, 15 mm moves the wheel 15 mm further out. That does two things at once: it reduces inboard clearance by very little and increases how far the wheel pokes toward the fender lip. A wider wheel also splits its extra width between in and out based on offset.

The useful rule when comparing a new wheel to your stock one: work out the change in how far the outer face and inner face move. A wheel that is wider and lower-offset can poke a lot more than the width alone suggests.

Will it clear? A practical approach

1. Note your factory wheel's width and offset as the baseline.
2. For the new wheel, compare how far the inner edge moves toward the strut (risk of rubbing suspension) and how far the outer edge moves toward the fender (risk of poke).
3. Account for tyre width — a wider tyre bulges beyond the rim on both sides.
4. Remember that lowering the car pulls the tyre up into the narrower part of the arch, so clearance that works at stock height can rub when slammed.

Spacers and adapters

A spacer effectively makes the offset more negative — a 15 mm spacer on an ET45 wheel behaves like ET30. It is a valid way to fine-tune fitment or clear big brakes, but it does not change the bolt pattern; that needs an adapter. Always check you retain enough stud thread engagement.