235/40 R18 vs 205/65 R15

205/65 R15 stays within the configured tolerance for 235/40 R18: +0.4% diameter and +0.4% speedometer effect.

Geometric comparison

205/65 R15 stays within the configured tolerance for 235/40 R18: +0.4% diameter and +0.4% speedometer effect.

Geometric comparison
Metric	This size: 235/40 R18	Alternative: 205/65 R15	Difference	Verdict
Overall diameter	25.40 in	25.49 in	+0.4%	Within tolerance
Section width	9.25 in	8.07 in	-1.18 in	Computed difference
Sidewall height	3.70 in	5.25 in	+1.55 in	Computed difference
Circumference	79.80 in	80.09 in	+0.4%	Within tolerance
Speedo	0.0%	+0.4%	+0.4%	Within tolerance

Compare sizes

Current tireLooks valid

New tireLooks valid

Market

Units

High fitment riskWithin tolerance · Diameter +0.4% (+0.09 in) · US (~3%)

High fitment risk · This rating covers tire geometry only, not vehicle compatibility.
Diameter +0.4% · Ride-height change +0.05 in.
Speedometer reads low · 60 mph → 60.2 mph.
Sidewall +1.55 in · Width -1.18 in.

Diameter+0.4%+0.09 in

Speedo @ 60+0.4%60 mph → 60.2 mph

Sidewall+1.55 inWidth -1.18 in

Ride-height change+0.05 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: 25.40 in 25.49 in+0.09 in
Sidewall: 3.70 in 5.25 in+1.55 in
Width: 9.25 in 8.07 in-1.18 in
Circumference: 79.80 in 80.09 in+0.4%
Revs / mile: 794 791
Speedo @ 60: 60 mph 60.2 mph+0.4%
Ride-height change: 0.00 in +0.05 in
True distance per 100 indicated: 100 mi 100.4 mi+0.4%

Current — 235/40 R18
New — 205/65 R15
Grounded to the road: the axle rises +0.05 in.

Result report

235/40 R18 205/65 R15 · High fitment risk · Within tolerance

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Markdown link

[235/40 R18 vs 205/65 R15 Tire size comparison](https://tirefitlab.com/tire-size-calculator/?from=235%2F40+R18&to=205%2F65+R15&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 `235/40R18`	+0.0%	+0.0%

235/40 R18 · 205/65 R15 · Methodology

Technical check: 2026-06-28

What changed

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

Methodology