Tire & wheel tools
Wheel offset (ET) calculator
Enter the old and new wheel width (inches) and ET (mm), plus an optional spacer, to see how far the outer edge moves and whether inner clearance grows or shrinks — with an ET↔backspacing converter.
New wheel pokes out
+22.7 mm
Inner clearance shrinks by +2.7 mm
The new outer edge moves +22.7 mm; inner clearance changes -2.7 mm (positive means more room).
| Old wheel inner position | 146.6 mm |
|---|---|
| Old wheel outer position | 56.6 mm |
| Old wheel backspacing | 159.3 mm |
| New wheel inner position | 149.3 mm |
| New wheel outer position | 79.3 mm |
| New wheel backspacing (pre-spacer) | 162.0 mm |
ET ↔ backspacing converter
| Result | |
|---|---|
| Width + ET → backspacing | 159.3 mm |
| Width + backspacing → ET | — |
Backspacing here uses the common convention: measured from the mounting pad to the inner rim edge including roughly 0.5in of lip, so BS = (width + 1) × 25.4 ÷ 2 + ET. Makers differ on whether stated width includes the lips — measure the real wheel when it matters.
How it is calculated
Inner position = wheel width (mm) ÷ 2 + ET − spacer; outer position = width ÷ 2 − ET + spacer, both measured from the mounting face. Poke = new outer − old outer; clearance change = old inner − new inner.
A higher ET tucks the wheel deeper into the arch, a lower ET pushes it out; a spacer behaves like a smaller ET. Same-width ET45→ET35 moves the outer edge out a theoretical 10mm and frees 10mm inside.
Outputs are theoretical wheel positions relative to the mounting face — not the real gap left on a car. Arch, suspension and caliper clearances need a physical check.
How to fill each field
- Wheel widthNominal width is bead-seat to bead-seat; the ~0.5in lips on each side are excluded. The backspacing converter adds back 1in under the common convention.
- ETPositive ET puts the mounting pad toward the outer face; negative toward the inside. Typical passenger wheels run ET35–ET50.
- SpacerA spacer pushes the whole wheel out: the outer edge gains the spacer thickness and the inner edge clears the same amount. It counts on the new wheel only.
FAQ
How far does ET45 to ET35 move the wheel?
At the same width, outer position = width÷2 − ET, so dropping 10mm of ET pushes the outer edge out 10mm and frees 10mm inside.
How do I convert ET and backspacing?
Under the common convention BS ≈ (width+1) × 25.4 ÷ 2 + ET and ET ≈ BS − (width+1) × 25.4 ÷ 2, all in mm. An 8J ET45 works out to about 6.29in (159.7mm).
Will the extra poke rub the fender?
This page only gives the theoretical shift relative to the mounting face — it cannot judge the remaining gap on a specific car. Verify by measurement when the edge nears the fender or suspension.
Is the spacer counted on both wheels?
Only on the new one. If the old setup also runs a spacer, subtract that thickness from the old ET before entering it.