Compare tire sizes

What changes when you change tire size?

A larger overall diameter raises the car and makes the speedometer read low; a smaller one lowers it and reads high. TireFitLab quantifies both effects and checks whether the difference stays within the selected tolerance.

Current tireLooks valid

New tireLooks valid

Market

Units

Requires verificationWithin tolerance · Diameter +1.7% (+0.43 in) · US (~3%)

Requires verification · This rating covers tire geometry only, not vehicle compatibility.
Diameter +1.7% · Ride-height change +0.21 in.
Speedometer reads low · 60 mph → 61.0 mph.
Sidewall -0.29 in · Width +0.39 in.

Diameter+1.7%+0.43 in

Speedo @ 60+1.7%60 mph → 61.0 mph

Sidewall-0.29 inWidth +0.39 in

Ride-height change+0.21 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: 24.97 in 25.40 in+0.43 in
Sidewall: 3.99 in 3.70 in-0.29 in
Width: 8.86 in 9.25 in+0.39 in
Circumference: 78.45 in 79.80 in+1.7%
Revs / mile: 808 794
Speedo @ 60: 60 mph 61.0 mph+1.7%
Ride-height change: 0.00 in +0.21 in
True distance per 100 indicated: 100 mi 101.7 mi+1.7%

Current — 225/45R17
New — 235/40R18
Grounded to the road: the axle rises +0.21 in.

Result report

225/45 R17 235/40 R18 · Requires verification · Within tolerance

Plan budget for this size: 235/40 R18

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Plan budget for this size

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<iframe src="https://tirefitlab.com/embed/tire-comparison/?from=225%2F45R17&to=235%2F40R18&market=us" title="225/45 R17 vs 235/40 R18 Tire size comparison" loading="lazy" width="720" height="330" style="width:100%;max-width:720px;border:0" referrerpolicy="strict-origin-when-cross-origin"></iframe>

Markdown link

[225/45 R17 vs 235/40 R18 Tire size comparison](https://tirefitlab.com/tire-size-calculator/?from=225%2F45R17&to=235%2F40R18&market=us)

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Budget$85–$120Per tire Mid-range$110–$160Per tire Premium$160–$225Per tire

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 `225/45R17`	+0.0%	+0.0%

Popular sizes

Content updated: 2026-07-16

What changed

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

Methodology