235/40 R18 vs 205/65 R15

What changes when you change tire size?

205/65 R15 stays within the configured tolerance for 235/40 R18: +0.4% diameter and +0.4% speedometer effect.

Geometric comparison

Geometric comparison
Metric This size: 235/40 R18 Alternative: 205/65 R15 Difference Verdict
Overall diameter 25.40 in 25.49 in +0.4% Within tolerance
Section width 9.25 in 8.07 in -1.18 in Computed difference
Sidewall height 3.70 in 5.25 in +1.55 in Computed difference
Circumference 79.80 in 80.09 in +0.4% Within tolerance
Speedo 0.0% +0.4% +0.4% Within tolerance

Compare sizes

Looks valid
Looks valid
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.

High fitment riskWithin tolerance · Diameter +0.4% (+0.09 in) · US preset (±3%)
  • High fitment risk · This rating covers tire geometry only, not vehicle compatibility.
  • Diameter +0.4% · Ride-height change +0.05 in.
  • Speedometer reads low · 60 mph → 60.2 mph.
  • Sidewall +1.55 in · Width -1.18 in.
Diameter+0.4%+0.09 in
Speedo @ 60+0.4%60 mph → 60.2 mph
Sidewall+1.55 inWidth -1.18 in
Ride-height change+0.05 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
25.40 in 25.49 in+0.09 in
Sidewall
3.70 in 5.25 in+1.55 in
Width
9.25 in 8.07 in-1.18 in
Circumference
79.80 in 80.09 in+0.4%
Revs / mile
794 791
Speedo @ 60
60 mph 60.2 mph+0.4%
Ride-height change
0.00 in +0.05 in
True distance per 100 indicated
100 mi 100.4 mi+0.4%
Δ⌀ +0.09 inΔ↔ -1.18 in
  • Current — 235/40 R18
  • New — 205/65 R15
  • Grounded to the road: the axle rises +0.05 in.

Tires and wheels in one comparison

Result report

235/40 R18 205/65 R15 · High fitment risk · Within tolerance

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

In-tolerance alternative sizes

In-tolerance alternative sizes
SizeDiameterSpeedo @ 100Use this
−1″ 235/45R17-0.3%-0.3%
OE 235/40R18+0.0%+0.0%
+1″ 235/35R19+0.3%+0.3%
+2″ 235/30R20+0.6%+0.6%

235/40 R18 · 205/65 R15 · Methodology