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Drivetrain

Wheel Offset & Fitment Calculator

Compare stock and aftermarket wheel width and offset (ET) to see how much the inner and outer edges move — toward the suspension or fender. Includes optional spacer thickness.

What this calculator is for

Wheel width and offset (ET) move the tire’s inner and outer edges relative to the hub. This calculator shows inboard and outboard change in millimeters when you go from stock wheels to aftermarket, including spacer thickness.

Essential before ordering $2k forged wheels for a lowered Mustang, F-150 with hub-centric spacers, or any flush-fender setup. A good outcome: you know whether the new combo tucks 5 mm closer to the strut or pushes 8 mm toward the fender lip.

Calculator

How to use this calculator

  1. Stock and new width are measured between bead seats (rim width in inches).
  2. Offset is ET in millimeters (positive ET = inward).
  3. Add spacer mm if you use wheel spacers — they move the wheel outward like lowering ET.

Width is rim width between beads, not overall wheel diameter. Tire section width affects rub before rim math does — this tool does not model tire bulge.

Spacers reduce effective ET (move wheel outward). Bolt-on adapters need their own safety checklist beyond offset math.

Always verify with the tire mounted and suspension loaded — camber and spring compression change clearance at ride height.

The math: do it without a calculator

Inboard change ≈ (stock ET − new ET − spacer) + (new width − stock width)÷2 × 25.4 mm

Positive inboard = wheel lip moves toward strut/coil. Always verify with the wheel and tire mounted.

Inboard change ≈ (stock ET − new ET − spacer) + (new width − stock width)÷2 × 25.4 mm.

Positive inboard = wheel moves toward suspension. Does not include hub face standoff differences between brands.

Real-world examples

Mustang 9″ wide +20 ET to 10.5″ +22 ET

Wider rim with slightly more positive ET often moves the outer lip outward a few mm while changing inboard clearance toward the strut — the calculator quantifies mm before you buy spacers to “make it fit.”

Troubleshooting & fine-tuning your setup

Calculator Says It Fits but Rubs on Turns

Offset math is rim-plane geometry only — tire sidewall bulge, camber, and spring compression at turn lock cause rubs the mm chart will not show. Test full lock both directions with passengers.

Spacers change effective ET and add stress on studs — verify length and torque specs beyond this worksheet.

Frequently asked questions

Wheel Offset & Fitment FAQs

Does lower ET stick out more?

Yes. Lower (more negative) ET moves the wheel outward toward the fender; higher ET tucks inward.

How much spacer equals ET change?

A 10 mm spacer roughly equals 10 mm less effective ET (wheel moves out). Include spacer mm in this calculator.

Will wider wheels rub even with correct offset?

Wider rims change inner and outer position even at same ET — the tool adds width half-change to both sides.

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