Speedometer error calculator

How do different tires affect your speedometer?

Your speedometer is calibrated for the original tire diameter. A larger diameter means you are actually going faster than indicated; a smaller one, slower. The error equals the percentage change in diameter — computed instantly below.

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%

More tools

Methodology

Technical check: 2026-06-28
Content updated: 2026-06-21

What changed

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

Methodology