Wheel offset and backspacing: one geometry, two units

A wheel's position relative to the hub is described two ways. European and original-equipment practice uses offset in millimetres; North American aftermarket practice uses backspacing in inches. They describe the same physical relationship from different reference points.

The two definitions

Offset is the distance from the wheel's mounting face — the flat surface that meets the hub — to the wheel's centreline.

• Positive offset: the mounting face sits outboard of the centreline. The wheel tucks in toward the vehicle.
• Zero offset: the mounting face is exactly on the centreline.
• Negative offset: the mounting face sits inboard. The wheel pushes out.

It is marked on the wheel as ET followed by a number, from the German Einpresstiefe, and a negative value is written ET-12.

Backspacing is the distance from the mounting face to the inner flange — the innermost edge of the rim. It is always positive, and it is measured in inches with a straightedge laid across the back of the wheel.

Converting between them

Backspacing depends on the wheel's width, and offset does not, which is the source of nearly every mistake:

backspacing (in) = (wheel width + 1) ÷ 2 + offset ÷ 25.4

The + 1 accounts for the flanges: a wheel's stated width is measured between the bead seats, and each flange adds roughly half an inch of overall width.

Worked both directions on a 9-inch wheel:

• At ET+18, backspacing is 5.71 in.
• At ET-12, backspacing is 4.53 in.
• A 9-inch wheel with 4.5 in of backspacing works out to about ET-13.

And on other widths: an 8.5-inch wheel at ET+35 gives 6.13 in of backspacing, while a 10-inch wheel at ET-25 gives 4.52 in. Note that the 9-inch ET-12 and the 10-inch ET-25 land at almost identical backspacing while sitting very differently on the vehicle — which is exactly why quoting one number without the width is meaningless.

What each number actually governs

They are not interchangeable in use, because they control clearance at opposite ends:

• Backspacing governs inboard clearance. More backspacing pulls the tire toward the suspension, the steering components, the frame rail and the brake caliper.
• Offset, read in the negative direction, governs outboard position. Less offset pushes the tire outward toward the fender lip and beyond the bodywork.

A wider wheel fitted at the same offset grows in both directions at once — half the extra width inboard, half outboard. This is the single most common surprise when moving from an 8-inch factory wheel to a 10-inch aftermarket one: the fender clearance problem people expected appears, and so does an inboard clearance problem they did not.

Measuring backspacing on a wheel you already own

The measurement needs a straightedge and a ruler, and it is taken with the wheel off the vehicle and lying face down:

1. Lay the straightedge across the inner flange, spanning the back of the wheel.
2. Measure from the underside of the straightedge to the mounting face — the machined surface the hub meets.
3. That distance is the backspacing.

Do it with the tire mounted and you will measure to the tire's bulge instead of the flange, which is why the tire comes off first. Offset is not directly measurable in the same way; it is derived from the backspacing and the width using the formula above, or read from the casting.

Scrub radius, which is why offset is not a free choice

The steering axis passes through the upper and lower ball joints (or the strut mount and lower joint) and continues to the ground. Scrub radius is the distance between where that line meets the road and the centre of the tire's contact patch.

Change offset and you move the contact patch relative to a steering axis you did not move. The consequences are steering feel, kickback over bumps, the loads the wheel bearing sees, and how the vehicle behaves under uneven braking. Wheel spacers do the same thing, by the same amount, for the same reason.

None of that is arithmetic this site can compute for you: it depends on the vehicle's suspension geometry, not on the wheel in isolation.

Spacers and adapters do the same arithmetic

A spacer of thickness t reduces effective offset by exactly t and reduces backspacing by the same amount converted into inches. A 25 mm spacer on an ET+45 wheel produces the geometry of an ET+20 wheel — identical numbers, identical scrub radius consequence, one additional joint in the load path and a stud engagement question that the spacer's own design has to answer.

An adapter, which changes bolt pattern as well, is a spacer with a second bolt circle machined into it and behaves the same way dimensionally.

Neither changes the tire's geometry, so nothing this site computes moves — but both change where that geometry sits relative to the bodywork, which is usually the constraint that was binding.

The other dimensions that have to match

Offset and backspacing are the two that get discussed. Three more decide whether the wheel fits at all:

• Bolt pattern — count and pitch circle diameter, quoted as 6x139.7 or 6x5.5.
• Centre bore — the hole in the middle. Hub-centric fitment carries the vehicle's weight on that bore rather than on the studs.
• Brake clearance — the inner profile of the spokes against the caliper, which is what stops many plus-sized packages going the other way.

How this interacts with tire size

Tire section width and wheel offset combine to decide where the widest point of the casing sits. A wider tire on the original wheels grows symmetrically about the wheel's centreline; the same tire on a lower-offset wheel moves outward as well.

For the tire half of that arithmetic, rim width to tire width covers how the wheel's width changes the tire's own width, and section width versus tread width covers which width is the one that meets an obstacle. The wheel half is a tape measure and your own vehicle, and there is no substitute for either.