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.

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Units
Editorial preset

These are TireFitLab editorial diameter-screening presets. Regional names choose a preset, not a national legal limit, ETRTO rule or vehicle approval. The AWD preset does not establish an allowable drivetrain mismatch. Use the exact vehicle and tyre manufacturer instructions; nominal geometry alone cannot approve fitment.

Requires verificationWithin tolerance · Diameter +1.7% (+0.43 in) · US preset (±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%
Δ⌀ +0.43 inΔ↔ +0.39 in
  • Current — 225/45R17
  • New — 235/40R18
  • Grounded to the road: the axle rises +0.21 in.

Tires and wheels in one comparison

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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Estimated budgetBudget$85–$120Per tireMid-range$110–$160Per tirePremium$160–$225Per tire

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

In-tolerance alternative sizes

In-tolerance alternative sizes
SizeDiameterSpeedo @ 100Use this
−1″ 225/50R16-0.5%-0.5%
OE 225/45R17+0.0%+0.0%
+1″ 225/40R18+0.5%+0.5%
+2″ 225/35R19+0.9%+0.9%

Popular sizes