Phase 7 · Asset & Tech Architecture

Wheel Offset & Tire Fitment Calculator

Before you drop $3,000 on wheels, know if they'll clear. Pit a new setup against stock and watch poke, inner clearance and speedometer error solve in real time.

Estimate — verify physically. Real clearance depends on suspension, ride height, camber, brake size and bodywork this model can't see. Use these numbers to screen a setup, then confirm fitment with a test fit or a trusted shop before buying.

How do you calculate wheel offset and fitment?

Offset (ET) is the distance in millimetres from a wheel's mounting face to its centreline. To check fitment, compare the new wheel against stock: outer edge (poke) = (wheel width ÷ 2) − offset, inner edge (inset) = (wheel width ÷ 2) + offset. A lower offset pushes the wheel out toward the fender; a higher offset moves the inner edge toward the strut and caliper.

Wheel widths are quoted in inches and offset in millimetres, so widths are converted (× 25.4) before the two setups are compared edge to edge.
Tire diameter = wheel diameter × 25.4 + 2 × (tire width × aspect ratio %). Keeping the new diameter near stock keeps your speedometer and gearing honest.
Speedometer error = (new rolling diameter − stock diameter) ÷ stock diameter. A taller overall tire means you are travelling faster than the dial reads.
This is geometry only — suspension travel, camber, brake size and bodywork decide the final millimetres, so always test fit before buying.

Stock vs new setup

Ten levers. Clearance and speedo error re-solve on every tick.

Stock — wheel diameter 17 in

Your current wheel diameter.

Stock — wheel width 7.5 in

Current wheel width.

Stock — offset (ET) 45 mm

Current offset in mm.

Stock — tire width 225 mm

Current tire section width.

Stock — tire aspect 45%

Current sidewall aspect ratio.

New — wheel diameter 18 in

Proposed wheel diameter.

New — wheel width 9.5 in

Proposed wheel width.

New — offset (ET) 35 mm

Proposed offset in mm.

New — tire width 255 mm

Proposed tire section width.

New — tire aspect 35%

Proposed sidewall aspect ratio.

Outer edge change (poke)

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How much further the wheel sticks out.

Inner edge change

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Rolling diameter change

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Speedo at indicated 60

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New tire diameter

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Under the hood

The math, fully exposed

Wheel widths convert to millimetres (×25.4), then we compare the two setups edge to edge:

Poke (outer edge) = (wheel width ÷ 2) − offset

Inset (inner edge) = (wheel width ÷ 2) + offset

Tire diameter = wheel diameter × 25.4 + 2 × (tire width × aspect%)

Speedometer error = (new diameter − stock diameter) ÷ stock diameter

Poke vs inset: a lower offset moves the outer edge toward the fender; a wider wheel or higher offset moves the inner edge toward the strut and caliper. Both are clearances you can run out of.
Diameter drives the speedo: keep the new tire's overall diameter close to stock and your speedometer and gearing stay honest. Big jumps throw both off.
Edges, not guarantees: this compares geometry only. Suspension travel, camber and bodywork decide the final millimetres — always test fit.

Your directives

What to do next, based on your numbers

Adjust the sliders to generate tailored recommendations.

Answers

Frequently asked questions

What is wheel offset (ET) and "poke"?

Offset (ET, in millimetres) is the distance from the wheel's mounting face to its centreline. A lower offset pushes the wheel outward toward the fender ("poke"); a higher offset tucks it inward. Poke is how far the outer edge sticks out compared to stock: (wheel width ÷ 2) − offset. More poke means a more aggressive stance — and a higher chance of rubbing the fender.

Will a wider wheel or tire rub?

It depends on which way the extra width goes. A lower offset moves the outer edge toward the fender (watch for fender rubbing, especially on bumps or full lock); a higher offset or wider wheel can move the inner edge toward the strut, spring or brake caliper. This tool shows both the outer (poke) and inner (inset) change so you can see which clearance is at risk.

How does changing tire size affect my speedometer?

Your speedometer is calibrated to the stock tire's rolling diameter. A taller overall tire travels further per revolution, so you're actually going faster than the dial reads (and your odometer under-counts); a shorter tire does the opposite. We compute the exact percentage error and what your true speed is when the dash shows 60.

Do I need spacers or hubcentric rings?

Spacers push the wheel outward to fine-tune clearance or stance, and any aftermarket wheel that isn't a perfect hub match needs hubcentric rings so the wheel centres on the hub rather than the bolts — this prevents vibration. If your setup adds significant poke, plan for fender work and proper hubcentric hardware rather than forcing the fit.

Open the full Wheel Offset & Fitment calculator on EmpireCalc →