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Inner edge{{ resultsReady ? shortSignedMm(computation.values.inner_clearance_change_mm) : '—' }} Outer edge{{ resultsReady ? shortSignedMm(computation.values.outer_position_change_mm) : '—' }} Verification{{ verificationBadge }}
Suspension Fender Current Proposed
The default is 8 in; changing units preserves the same physical width.
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Keep the ET sign supplied by the wheel manufacturer.
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The default is 9 in; use the exact proposed wheel specification.
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Positive and negative offsets move the mounting plane in opposite directions.
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Blank is neutral. Measure at the relevant steering, load, and suspension state.
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Blank is neutral. Measure to the nearest body or fender obstruction.
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Record the smallest direct gap for the exact proposed wheel.
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A static observation does not cover untested steering or suspension travel.
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Record the smallest checked gap; blank is neutral.
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Confirm bolt pattern, centre bore, fasteners, load rating, brake barrel and spoke clearance, the exact tire envelope, body and suspension clearance, full steering lock, full suspension travel, manufacturer requirements, and local rules before purchase or use.
Calculation method:
Nominal rim width does not include exact flange, spoke, tire, or barrel geometry.
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The chart renderer is unavailable. The same signed movements remain available in the fitment plan and verification ledger.

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Changing wheel width or offset moves both rim edges even when tire diameter stays the same. The inward edge affects room near suspension parts and the wheel well. The outward edge affects fender coverage, body clearance, and how far the wheel appears to poke. Comparing diameter alone misses both movements.

Wheel offset, often marked ET, is the distance from the wheel centerline to the hub mounting face. Positive offset places the mounting face toward the outside face of the wheel and moves the wheel inward. Lowering the offset moves the wheel outward. A width increase is split between the inner and outer edges before the offset difference is applied.

  • More inner clearance means the proposed nominal inner rim edge moves away from the vehicle.
  • Less inner clearance means that edge moves closer to suspension or body components.
  • More outer position means the nominal outer rim edge moves farther toward the fender.
  • Less outer position means the edge tucks farther inward.

These movements are useful for screening a proposed wheel specification against a known setup. They are not a complete fitment decision. Nominal rim width does not describe spoke shape, barrel profile, flange thickness, tire section width, sidewall bulge, tire growth, hub bore, bolt pattern, load rating, fastener seat, or brake-caliper envelope.

Clearance also changes while the vehicle moves. Steering angle, suspension compression and droop, alignment, load, body flex, tire pressure, and manufacturing tolerances can reduce a gap that looks adequate while parked. Use calculated edge movement to plan measurements, then physically verify the exact wheel and tire throughout the relevant range of motion.

How to Use This Tool:

Compare the current and proposed nominal wheel specifications on the same axle and preserve the manufacturer's offset sign.

  1. Enter the current nominal width and wheel offset. Choose inches or millimetres for width and preserve the ET sign printed by the wheel manufacturer.
  2. Enter the proposed nominal width and offset. Unit changes preserve the same physical measurement, so check the converted number before comparing specifications.
  3. Add measured current inner or outer gaps when you want projected nominal clearances. Leave a gap blank when it was not measured.
  4. Record direct proposed-wheel gaps to the brake, suspension, and tire-to-body obstruction only after checking the exact parts. Use the smallest observed gap.
  5. Review the inner-clearance change, outer-position change, projected gaps, and verification status. Any zero or negative recorded or projected clearance is a stop-and-investigate result, while positive gaps still require physical verification.

Interpreting Results:

A positive inner-clearance change means more nominal space on the inside; a negative value means the proposed inner edge moves closer to the vehicle by that amount. A positive outer-position change means additional poke, while a negative value means the proposed outer edge tucks inward.

StatusMeaningFollow-up
Physical verification requiredThe nominal movement is known, but no usable gap evidence was entered.Measure the exact wheel, tire, brake, suspension, and body clearances.
Positive gaps recorded, not certifiedEvery entered or projected gap is above zero.Repeat checks at steering lock, load, compression, droop, and the required safety margin.
Nonpositive clearance recordedAt least one entered or projected gap is zero or below zero.Do not treat the specification as fitting until the conflict and measurement basis are resolved.

Technical Details:

All dimensions are normalized to millimetres, using 25.4 mm per inch. Nominal width W is divided equally around the wheel centerline. Offset E then shifts the mounting face relative to that centerline, producing an inner position W/2 + E and an outer position W/2 − E.

Formula Core:

Subscripts c and p denote current and proposed wheels. Positive ΔCinner means gained inner clearance; positive ΔPouter means increased outward position.

Ic=Wc2+Ec,Ip=Wp2+Ep ΔCinner=Ic−Ip Oc=Wc2−Ec,Op=Wp2−Ep ΔPouter=Op−Oc Cprojected inner=Cmeasured inner+ΔCinner Cprojected outer=Cmeasured outer−ΔPouter

For example, changing from a 7-inch ET42 wheel to an 8-inch ET32 wheel moves the nominal inner edge outward by 2.7 mm, gaining that much inner clearance. The outer edge moves 22.7 mm farther toward the fender. These values use nominal bead-seat width rather than exact flange or tire dimensions.

Supported nominal widths are 3 to 20 inches, offsets are −150 to 150 mm, and optional gaps are 0 to 500 mm. Calculations retain full precision; displayed rounding does not create a clearance margin.

Fitment Limitations:

Nominal edge geometry cannot certify wheel or tire fitment, road legality, load suitability, or safe installation.

  • Confirm bolt pattern, center bore, fastener type and engagement, load rating, tire compatibility, and manufacturer requirements.
  • Use a brake-clearance template or a direct test fit for spoke and barrel clearance; width and offset do not describe those shapes.
  • Check tire-to-body and suspension gaps at full steering lock and across suspension travel with representative load and alignment.
  • Have a qualified installer review the complete wheel, tire, spacer, hub, brake, and vehicle combination when safety depends on the result.

References: