Wheel offset vs backspacing
Both describe where the mounting face sits. Offset measures from the wheel's centreline in millimetres; backspacing measures from the inner rim edge in inches. Neither is more correct — they are different conventions.
Converting between them
backspacing (in) = (wheel width in inches ÷ 2) + 0.5 + (offset in mm ÷ 25.4)
The extra half inch accounts for the rim flanges, which sit outside the quoted wheel width — a "8J" wheel measures about 9 inches across its flanges. Omitting it is the single most common error in this conversion.
Worked example
An 8-inch wheel at ET45: half the width is 4 inches, plus 0.5 for the flanges, plus 45 ÷ 25.4 = 1.77 inches. Backspacing is 6.27 inches.
Going the other way, a 8-inch wheel with 5 inches of backspacing gives offset = (5 − 4 − 0.5) × 25.4 = 12.7 mm, so roughly ET13.
Conversion table
| Wheel width | ET0 | ET15 | ET30 | ET45 | ET60 |
|---|---|---|---|---|---|
| 7J | 4.00" | 4.59" | 5.18" | 5.77" | 6.36" |
| 7.5J | 4.25" | 4.84" | 5.43" | 6.02" | 6.61" |
| 8J | 4.50" | 5.09" | 5.68" | 6.27" | 6.86" |
| 8.5J | 4.75" | 5.34" | 5.93" | 6.52" | 7.11" |
| 9J | 5.00" | 5.59" | 6.18" | 6.77" | 7.36" |
| 9.5J | 5.25" | 5.84" | 6.43" | 7.02" | 7.61" |
| 10J | 5.50" | 6.09" | 6.68" | 7.27" | 7.86" |
Read across a row to see how offset changes backspacing at a fixed width, and down a column to see how width changes it at a fixed offset. Both move it, which is exactly why the two figures cannot be compared without knowing the width.
Why backspacing hides a trap
Backspacing depends on wheel width, so the same backspacing figure means different things on different widths. Two wheels both at 5 inches backspacing — one 7 inches wide, one 9 — sit 25 mm apart in the arch. Offset does not have this problem, which is why it is the modern standard and what the fitment calculator uses.
Related
How are these fitment figures calculated?
Fitment North calculates declared wheel and tyre geometry from the dimensions shown on the page, using the same tested method as the wheel-offset and tyre-size calculator. Tyre diameter uses rim diameter plus two calculated sidewalls; offset comparisons split width changes across inner and outer edges before applying the offset difference. Results are measurement aids, not vehicle approval. Confirm ratings, clearances and manufacturer guidance before fitting parts.