225/45 R17 vs 225/40 R18

Can 225/40R18 replace 225/45R17?

225/40R18 keeps the nominal 225 mm width while changing the wheel from 17 to 18 inches. Diameter rises only 2.9 mm (+0.46%), but sidewall height falls 11.25 mm. A near diameter match does not make the tires interchangeable on the same wheel: an 18-inch wheel is required. Enter its width and ET below to compare edge positions. Verify the exact tire’s approved rim range, load and speed ratings, physical clearance and the vehicle manufacturer’s requirements.

Geometric comparison

Geometric comparison
Metric This size: 225/45 R17 Alternative: 225/40 R18 Difference Verdict
Overall diameter 24.97 in 25.09 in +0.5% Within tolerance
Section width 8.86 in 8.86 in 0.00 in Computed difference
Sidewall height 3.99 in 3.54 in -0.44 in Computed difference
Circumference 78.45 in 78.81 in +0.5% Within tolerance
Speedo 0.0% +0.5% +0.5% Within tolerance

Compare sizes

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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.

Minor variationWithin tolerance · Diameter +0.5% (+0.11 in) · US preset (±3%)
  • Minor variation · This rating covers tire geometry only, not vehicle compatibility.
  • Diameter +0.5% · Ride-height change +0.06 in.
  • Speedometer reads low · 60 mph → 60.3 mph.
  • Sidewall -0.44 in · Width +0.00 in.
Diameter+0.5%+0.11 in
Speedo @ 60+0.5%60 mph → 60.3 mph
Sidewall-0.44 inWidth +0.00 in
Ride-height change+0.06 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 25.09 in+0.11 in
Sidewall
3.99 in 3.54 in-0.44 in
Width
8.86 in 8.86 in+0.00 in
Circumference
78.45 in 78.81 in+0.5%
Revs / mile
808 804
Speedo @ 60
60 mph 60.3 mph+0.5%
Ride-height change
0.00 in +0.06 in
True distance per 100 indicated
100 mi 100.5 mi+0.5%
Δ⌀ +0.11 inΔ↔ +0.00 in
  • Current — 225/45 R17
  • New — 225/40 R18
  • Grounded to the road: the axle rises +0.06 in.

Tires and wheels in one comparison

Result report

225/45 R17 225/40 R18 · Minor variation · Within tolerance

Plan budget for this size: 225/40 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%

225/45 R17 · 225/40 R18 · Methodology