Wheel Offset and Backspacing
Converting wheel offset to backspacing, and where the inch hides.
Example
You enter
- Rim (bead-seat) width (in) 8
- Offset ET (mm, + = outboard face; use if solving from offset) 45
You get
- Overall width (bead seat + 1 in) 9.00
- Backspacing 6.27 in
- Frontspacing 2.73 in
Checked against published worked examples from: Speedway Motors (J. McCollough).
Details, formula, and sources
ScopeOffset (ET, mm, mounting face to centerline) and backspacing (in, mounting face to inboard rim edge) describe the same fitment in different units. backspacing = rim_width/2 + 0.5 + offset/25.4, and the rim 'width' is the bead seat -- the wheel is ~1 in wider overall (half an inch per flange), the omission that makes a fitment come out an inch wrong. An 8 in wheel at +45 mm has 6.27 in backspacing and 2.73 in frontspacing; a zero-offset wheel sits nearly 1.8 in further out. A more positive offset pulls the wheel inboard (more fender, less brake/strut clearance). A fitment aid; the wheel, hub, and suspension clearances govern.
Formula or tableoverall_width = rim_width + 1; backspacing = rim_width/2 + 0.5 + offset_mm/25.4; offset_mm = (backspacing - rim_width/2 - 0.5) x 25.4; frontspacing = overall_width - backspacing.
Source and editionWheel offset / backspacing conversion (Tire & Rim Association wheel dimensions), first-principles geometry, by name; the actual wheel, hub, and suspension clearances govern.
Free accessThe offset-backspacing conversion is first-principles wheel geometry; the rim width and offset come from the wheel specification.
What governsEstimate. AHJ and licensed professional govern.
API field namesrim_width_in, offset_mm, overall_width_in, backspacing_out_in, frontspacing_in
- Bead seat vs overall the rim width is the bead seat; the wheel is ~1 in wider overall (0.5 in per flange)Tire & Rim Association
- Positive offset a more positive offset pulls the wheel inboard (more fender, less brake/strut clearance)wheel fitment practice
- Same geometry backspacing and offset describe the same fitment in different units and directionsgeometry