buncha/engineering/wheel-offset

Wheel Offset Calculator

Plan a wheel swap before you buy. Plug in your current setup and the new wheels you're eyeing — get the inboard / outboard distance, fender poke, rolling-diameter change, speedo error, camber-adjusted clearance risk, and a live cross-section showing how the new wheel sits relative to the suspension. Edit the inputs on the left and watch the diagram update in real time on the right.

All math runs in your browser. Nothing is uploaded.

Existing setup

Diameter (in)

Width (in)

Offset (mm, ET)

Spacer (mm)

Tire width (mm)

Tire profile (%)

Camber (°)

Tire fit profile

× 1.00 section width

235/40R18 · 8J ET+45

New wheels

Diameter (in)

Width (in)

Offset (mm, ET)

Spacer (mm)

Tire width (mm)

Tire profile (%)

Camber (°)

Tire fit profile

× 1.00 section width

255/35R19 · 9.5J ET+25

Cross-section

Click the Existing / New chips above to show or hide each setup. Suspension shown for context only — real clearance depends on your car's geometry and trim.

At a glance

Poke

+39.0 mm

Further out toward the guard

Inset

-1.0 mm

More strut clearance

Diameter

+15.9 mm

Ride-height +7.9 mm

Speedo

-2.5%

Speedo reads slow vs actual

Your results

	Existing	New
Wheel + tyre
Diameter	645.2 mm	661.1 mm
Circumference	2027.0 mm	2076.9 mm
Fitment
Poke (outboard)	56.6 mm	95.6 mm
Inset (inboard)	146.6 mm	145.6 mm
Tyre overhang	15.9 mm	6.9 mm
Outer tyre edge	72.5 mm	102.5 mm
Inner tyre edge	162.5 mm	152.5 mm
On the road
Speedo reading at 30 mph	30.0 mph	29.3 mph
Speedo reading at 60 mph	60.0 mph	58.6 mph
Speedo error	0.0%	-2.5%
Ride height	322.6 mm	330.5 mm
Arch gap change	baseline	-7.9 mm

Plain English

The new wheel's inner rim sits 1.0 mm further from the suspension.

The outer rim pokes out 39.0 mm more than before.

Tyre shoulders move further: inner edge by -10.0 mm and outer edge by +30.0 mm after the section-width bulge.

Rolling diameter changes by +15.9 mm — that's a ride-height change of about +7.9 mm. Your speedometer will read 2.5% slow.

Geometric estimate only. Real fitment depends on tyre brand, shoulder shape, brake clearance, guard shape, alignment, steering angle, load, body roll and manufacturing tolerances. Always physically check clearance before driving.

Wheel reference

Existing setup

235/40R18 · 8J ET+45 · Camber 0°

New setup

255/35R19 · 9.5J ET+25 · Camber 0°

What the numbers mean

Offset (ET).Distance in mm from the hub mounting face to the wheel’s centerline. Positive ET pulls the wheel inward (more tuck). Negative ET pushes it out (more poke). ET0 = mount face is at the wheel centerline.

Inboard distance.Hub mount face to the inner edge of the wheel. Eats into your strut / control-arm / inner-fender clearance. Bigger than the original means tighter inner clearance.

Outboard distance.Hub mount face to the outer edge of the wheel. Controls how much the wheel pokes past the fender. Bigger than the original means more poke.

Camber.Vertical wheel tilt. Negative camber tilts the top inward (track / aggressive fitment look). Affects the horizontal position of the contact patch — calculator shows the shift in mm relative to the mount face.

Tire OD (rolling diameter).Wheel size + 2× sidewall. A change >3% from stock will confuse the speedometer and ABS. Calculator flags swaps that move OD by more than 20 mm.

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About Wheel Offset Calculator

Plan a wheel + tire swap before you buy. Inputs existing and new wheel specs (diameter, width, offset, tire size, camber) — outputs a side-by-side results table (poke, inset, circumference, speedo error, reading at 30/60 mph, ride-height gain, arch-gap loss), an edge-on cross-section diagram with both wheels overlaid on a sketched suspension strut, and a plain-English summary explaining exactly what the swap changes.

Everything happens on your device. Close the tab and it's gone.