Offset and backspacing
Offset is the signed distance from the wheel centerline to its mounting face, in millimeters. Positive offset moves the centerline inward; negative offset moves it outward. Backspacing is measured from the mounting face to the inner rim lip, not the bead seat.
With bead width W and total lip allowance A in inches, overall rim width is W + A. Converted offset = [backspacing − (W + A) / 2] × 25.4 mm. The conversion uses a symmetric lip assumption. Confirm the wheel manufacturer’s actual dimensions.
Outer envelope calculation
Outward center position = −offset / 25.4 + spacer. Outer envelope = outward center position + max(mounted tire section width, overall rim width) / 2. Per-side change is the new envelope minus the current envelope; axle span changes by twice that amount. If you supply another axle/body width, overall width is the larger span.
A wider bead seat alone does not add tire width twice. When signed offset and the supplied mounted tire width stay the same, tire edges stay in the same position. In reality, changing rim width can change mounted tire width: obtain or measure that new section width yourself.
Worked example
Current wheels: 7-inch bead width, +25.4 mm offset, 10-inch mounted tires and a 64-inch measured axle span. New wheels: 8-inch bead width, 0 mm offset and the same entered 10-inch mounted tire width. With a 1-inch lip allowance and no spacers, the envelope moves outward 1 inch per side. Estimated modeled span is 66 inches. A wider body or other axle could still control overall vehicle width.
Can I enter a 4+3 marking?
Do not assume that a split marking describes measured outer-lip backspacing. Obtain the wheel maker’s offset or backspacing definition and dimensions. Enter signed millimeters or measured backspacing in inches.
Does this guarantee trailer or trail fit?
No. Check the completed vehicle’s widest point, gate/door opening, height and usable deck, including obstructions and tie-down access. The linked trailer tool receives a calculated estimate, which you can replace with an actual measurement.
Source and limitations
The offset/backspacing definitions and conventional 1-inch lip adjustment follow the Mickey Thompson technical bulletin (reviewed October 5, 2026). That general wheel reference does not verify any UTV wheel or fitment. Read our methodology and source policy.