Wheel Offset & Backspacing Geometry

Method and units

Use nominal bead-seat rim width in inches, not lip-to-lip width, and convert it to millimeters. Nominal inboard distance = half-width + offset; outboard distance = half-width − offset. Backspacing is nominal inboard distance converted to inches. Change in inner clearance = old inboard distance − new inboard distance; positive means more nominal clearance. Change in outer extension = new outboard distance − old outboard distance; positive means more poke.

Worked example

A 7.5-inch wheel at +35 mm has 130.25 mm nominal inboard and 60.25 mm outboard. An 8-inch wheel at +25 mm has 126.6 mm inboard and 76.6 mm outboard: about 3.7 mm more inner clearance and 16.4 mm more outer extension.

Supported inputs and interpretation

Rim widths 4–15 in; offsets −100 to +150 mm. Results are rounded for display only; internal calculations retain full JavaScript precision. Entered measurements and assumptions determine the answer.

References and limitations

The equations here use the stated unit conversions and geometric or accounting relationships; no empirical vehicle limits or manufacturer presets are supplied.

This idealized bead-seat geometry excludes tire bulge, rim lips, camber, steering sweep, brakes and vehicle tolerances. Measure the actual assembly and confirm fitment with the vehicle/wheel manufacturer or a qualified professional. Geometry does not establish safe fitment or approval to modify or use a vehicle.

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This planner performs the stated arithmetic; it is not a vehicle inspection, engineering approval, manufacturer's recommendation or professional review.