235/55 R18 vs 275/40 R20

275/40 R20 stays within the configured tolerance for 235/55 R18: +1.7% diameter and +1.7% speedometer effect.

Geometric comparison

275/40 R20 stays within the configured tolerance for 235/55 R18: +1.7% diameter and +1.7% speedometer effect.

Geometric comparison
Metric	This size: 235/55 R18	Alternative: 275/40 R20	Difference	Verdict
Overall diameter	28.18 in	28.66 in	+1.7%	Within tolerance
Section width	9.25 in	10.83 in	+1.57 in	Computed difference
Sidewall height	5.09 in	4.33 in	-0.76 in	Computed difference
Circumference	88.52 in	90.04 in	+1.7%	Within tolerance
Speedo	0.0%	+1.7%	+1.7%	Within tolerance

Compare sizes

Current tireLooks valid

New tireLooks valid

Market

Units

High fitment riskWithin tolerance · Diameter +1.7% (+0.48 in) · US (~3%)

High fitment risk · This rating covers tire geometry only, not vehicle compatibility.
Diameter +1.7% · Ride-height change +0.24 in.
Speedometer reads low · 60 mph → 61.0 mph.
Sidewall -0.76 in · Width +1.57 in.

Diameter+1.7%+0.48 in

Speedo @ 60+1.7%60 mph → 61.0 mph

Sidewall-0.76 inWidth +1.57 in

Ride-height change+0.24 inDiameter ÷ 2

How this result is calculated

Computed from nominal tire-size geometry: diameter, circumference and speedometer error. It does not check vehicle placard approval, load/speed rating, rim-width range, wheel-arch, suspension or brake clearance.

Methodology

Before fitting

Check the vehicle placard and owner’s manual first.
Match or exceed the required load index and speed rating.
Confirm rim-width range, bolt pattern, hub bore, offset and fasteners.
Inspect wheel-arch, suspension and brake clearance under real vehicle load.
Check TPMS, cold pressure and local roadworthiness rules.

Diameter: 28.18 in 28.66 in+0.48 in
Sidewall: 5.09 in 4.33 in-0.76 in
Width: 9.25 in 10.83 in+1.57 in
Circumference: 88.52 in 90.04 in+1.7%
Revs / mile: 716 704
Speedo @ 60: 60 mph 61.0 mph+1.7%
Ride-height change: 0.00 in +0.24 in
True distance per 100 indicated: 100 mi 101.7 mi+1.7%

Current — 235/55 R18
New — 275/40 R20
Grounded to the road: the axle rises +0.24 in.

Result report

235/55 R18 275/40 R20 · High fitment risk · Within tolerance

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Markdown link

[235/55 R18 vs 275/40 R20 Tire size comparison](https://tirefitlab.com/tire-size-calculator/?from=235%2F55+R18&to=275%2F40+R20&market=us)

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Geometric estimate. Always verify physical clearance (arches, suspension, brakes) before fitting.

In-tolerance alternative sizes

In-tolerance alternative sizes
Size	Diameter	Speedo @ 100	Use this
OE `235/55R18`	+0.0%	+0.0%

235/55 R18 · 275/40 R20 · Methodology

Technical check: 2026-06-28

What changed

Checked generated routes, metadata, sitemap inclusion and localized page rendering.

Methodology