Compare two tire sizes › Wheel offset
Wheel offset: will new wheels stick out or sit further in?
Type the width and offset stamped on your current wheel and on the one you are thinking of (inside the spokes, e.g. 8Jx17 ET35 or 17x8 +35). Add the tire sizes to see the sidewalls move too.
Working it out…
Scrub radius and track
Drawn to scale
Every dimension
| Measure | Current | New | Difference |
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Positions are measured from the hub mounting face. Backspacing is how deep the wheel reaches in behind the hub face; front spacing is how far it reaches out in front of it. Many US wheel catalogues quote backspacing to the lip and an overall width 1 in wider than the nominal one; European ET figures always use the nominal (bead seat) width.
The formulas
A wheel marked 8Jx17 ET35 is 8 in wide between the bead seats (where the tire beads sit), has a 17 in bead-seat diameter, a J-shaped flange, and an offset of 35 mm. Offset (ET, from German Einpresstiefe) is the distance from the wheel's centre line to its mounting face. Positive ET means the mounting face sits outboard of the centre line, so the wheel tucks in; negative ET pushes it out. US stamps write the same thing as 17x8 +35.
half width = width (in) × 25.4 ÷ 2 backspacing = half width + ET to the lip = backspacing + 12.7 mm front spacing = half width − ET outer move = new front sp. − old front sp. inner move = new backsp. − old backsp. centre move = old ET − new ET (+ = out)
Worked through for 8.0J ET35: half width = 8.0 × 25.4 ÷ 2 = 101.6 mm (4.0 in); backspacing = 101.6 + 35 = 136.6 mm (5.38 in) to the bead seat, or 149.3 mm (5.88 in) to the lip; front spacing = 101.6 − 35 = 66.6 mm.
Going to 9.0J ET30: half width = 114.3 mm; front spacing = 114.3 − 30 = 84.3 mm, backspacing = 114.3 + 30 = 144.3 mm. The outer face moves 84.3 − 66.6 = 17.7 mm out and the inner face 144.3 − 136.6 = 7.7 mm in. Only the extra half inch of width on each side (12.7 mm) is shared; the 5 mm lower offset shifts the whole wheel outward, so the outside gains 12.7 + 5 and the inside 12.7 − 5.
Converting a US backspacing figure to ET works the same sum backwards. A 17x9 wheel sold with 5.5 in backspacing (to the lip): ET = 5.5 × 25.4 − 12.7 − 114.3 = +12.7 mm. A wheel spacer of t mm is the same as lowering ET by t: an 8J ET45 with a 10 mm spacer sits exactly where an 8J ET35 would.
With a tire: its sidewalls sit at the wheel centre line ± half the section width. A 225/45R17 on 8J ET35 reaches −35 + 112.5 = 77.5 mm out from the hub face and 147.5 mm in.
What this cannot do
- It gives no fitment verdict. Whether a wheel clears depends on the car: the distance from the hub face to the fender lip and to the strut, spring perch and brake caliper, the camber, how far the suspension compresses and the steering lock. None of that is in the wheel stamp. Measure your own clearances (with the car on its wheels and with the steering at full lock), then compare them to the moves shown here.
- Brake clearance is about the spoke shape, not only the offset. Two wheels with the same width, diameter and ET can differ in how far the spokes curve inward toward the caliper. Makers publish caliper templates for this; this page cannot see spoke profiles.
- Tires are drawn at their nominal section width. On a wider rim the same tire stretches wider, by roughly 5 mm per extra half inch of rim, and real tires bulge beyond the nominal figure near the ground under load. Treat the sidewall numbers as ± several mm.
- It does not check the rest of the wheel spec. Bolt pattern (PCD), centre bore, the bolt or nut seat shape (cone, ball, flat) and the wheel's load rating all have to match the car too, as does the car maker's approved list or the local inspection rules. EU Regulation 1009/2010, for example, requires the bodywork to cover the tire over a set arc ahead of and behind the top of the wheel.
- The scrub-radius note is a direction, not a value. The steering axis is set by the car's suspension, so this page can say how much the contact patch moves relative to it, not what the scrub radius is.