Tire size calculator

How do you compare two tire sizes?

Enter your current and new tire size. TireFitLab instantly shows the difference in overall diameter, sidewall and width, the speedometer error, and whether the swap 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

Copy/print uses this page state only; TireFitLab does not create an account or store a server-side report.

Plan budget for this size

Embed or cite this result

HTML iframe

<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)

Open embed preview

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%

Worked examples

These rows use the same geometry engine as the calculator, so the numbers match the interactive result above.

These rows use the same geometry engine as the calculator, so the numbers match the interactive result above.
Current size	Candidate size	Diameter delta	Speedometer at 60 mph	Tolerance
225/45R17	235/40R18	+1.7% (+10.9 mm)	60 mph → 61.0 mph	within
205/55R16	225/45R17	+0.4% (+2.4 mm)	60 mph → 60.2 mph	within
215/65R16	225/60R17	+2.3% (+15.9 mm)	60 mph → 61.4 mph	within

What to do with it: Use the exact vehicle placard size before buying.

Placard-first

Start from the size already approved for the car

Enter the tire size from the door placard, fuel flap or current sidewall, then compare a candidate replacement size.

Size-based only: no make/model database claim, and no physical-clearance guarantee.

Popular sizes

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Content updated: 2026-07-16

What changed

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