ElpiAutoWheel Offset Calculator
Will a new wheel poke past the fender, or sit closer to the strut? Enter both and see.
Your wheels
The new wheel pokes out 23 mm more, and sits 3 mm closer to the strut.
Backspacing 6.27″ → 6.38″ (+0.11″)
New wheel, cut through the middle
8J ET45 vs 9J ET35, measured
Both wheels bolt to the same hub face. Positive offset (ET) pulls the wheel inboard; a lower or negative number pushes it out.
| Measurement | 8J ET45 | 9J ET35 | Difference |
|---|---|---|---|
| Rim width | 8″ | 9″ | +1″ |
| Offset (ET) | 45 mm | 35 mm | −10 mm |
| Backspacing | 6.27″ | 6.38″ | +0.11″ |
| Outer edge → hub | 69 mm | 92 mm | +23 mm |
| Inner edge → hub | 159 mm | 162 mm | +3 mm |
Poke and inner-clearance figures fold in the tyre section width when you enter it — the tyre bulges wider than the rim, so a wider tyre adds poke on both edges even on the same wheel.
What offset does
Width, offset, and where the wheel sits
A wheel bolts to the hub at a fixed face. Offset (ET) is how far that face sits from the wheel's centreline, in millimetres. More positive offset pulls the wheel inboard, toward the strut; less offset, or a negative number, pushes it outboard, toward the fender. Width adds rubber on both sides at once — half an inch of extra width is about 6 mm on each edge.
So going to a wider wheel with a lower offset does two things at once: the outer edge moves toward the arch (poke), and the inner edge moves — toward the strut if the width gain outweighs the offset drop, away from it if not. The tool separates the two so you know which one to measure.
The math
How the numbers are worked out
outer edge → hub(mm) = ( widthin / 2 + 0.5 ) × 25.4 − offsetmminner edge → hub(mm) = ( widthin / 2 + 0.5 ) × 25.4 + offsetmm
backspacing(in) = width / 2 + 0.5 + offset / 25.4
The 0.5″ is the rim flange — the lip beyond the bead seat, one on each side. Poke is the change in the outer-edge figure between the two wheels; inner clearance is the change in the inner-edge figure. When you enter tyre section widths, half the difference is added to both, because the tyre bulges wider than the rim.
Common questions
What is wheel offset (ET)?
Offset is the distance in millimetres from the wheel's centreline to the hub mounting face. A higher (more positive) number pulls the wheel inboard, toward the strut; a lower or negative number pushes it outboard, toward the fender. ET is stamped on the back of the wheel, e.g. ET45. Two wheels with the same width but 10 mm less offset sit 10 mm further out.
Will a lower offset rub?
It can. Dropping offset moves the whole wheel toward the fender, so the tyre's outer edge is closer to the arch lip — most likely to catch at full lock or full suspension compression. It also moves the inner edge away from the strut, so inboard clearance improves. A wider wheel adds width on both edges at once. Enter both wheels above to see the exact millimetres.
Does a wheel spacer change offset?
Effectively yes. A spacer of thickness T sits between the hub and the wheel, so it pushes everything outboard by T mm — the same as reducing offset by T. It adds poke and moves the inner edge away from the strut. It cannot pull a wheel inboard, so a spacer only helps when the new wheel would otherwise tuck in too far.
How much offset change is safe?
There is no single number — it depends on how much arch and strut clearance the car has from the factory. As a rule of thumb, a few millimetres is unnoticeable, up to about 10 mm is usually fine on a car with normal clearance, and beyond that you should measure. The tyre section width matters as much as the wheel: a 20 mm wider tyre adds 10 mm of poke on its own.
What is backspacing?
Backspacing is the distance from the hub mounting face to the inner edge of the rim, in inches. It is another way of stating the same geometry as width and offset: backspacing = wheel width ÷ 2 + 0.5 + offset ÷ 25.4. A wheel with more backspacing tucks further in.
Methodology
Geometry uses the standard wheel model: overall rim width is the stated J figure plus half an inch of flange per side; the hub face sits offsetmillimetres outboard of the centreline. Poke and inner-clearance changes are the differences in the outer- and inner-edge distances between the two wheels, optionally shifted by half the tyre section-width difference. Real clearance also depends on arch shape, strut and caliper geometry and suspension travel — always confirm on the car, at full lock and full compression, before fitting.
Reviewed 2026-09-11.