TireFitLab

225/40 R18

What size is 225/40 R18?

225/40 R18 has an overall diameter of about 25.09 in, a section width of 8.86 in and a sidewall height of 3.54 in. It turns roughly 804 times per mile. Below are the exact figures plus plus/minus sizes that stay within the selected diameter tolerance.

Geometric comparison

255/35 R18 stays within the configured tolerance for 225/40 R18: -0.2% diameter and -0.2% speedometer effect.

Geometric comparison
Metric This size: 225/40 R18 Alternative: 255/35 R18 Difference Verdict
Overall diameter 25.09 in 25.03 in -0.2% Within tolerance
Section width 8.86 in 10.04 in +1.18 in Computed difference
Sidewall height 3.54 in 3.51 in -0.03 in Computed difference
Circumference 78.81 in 78.63 in -0.2% Within tolerance
Speedo 0.0% -0.2% -0.2% Within tolerance

Seasonal check

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What this geometry means

225/40 R18 has a low-profile proportion. The marking combines nominal section width 8.86 in, sidewall ratio 40% and rim diameter 18 inches. One sidewall is 3.54 in; both sidewalls make up 28% of the 25.09 in overall diameter, while the rim makes up 72%.

The closest diameter entry is 255/35 R18 at -0.2%. Relative to 225/40 R18, it changes nominal section width by +1.18 in, each sidewall by -0.03 in and radius by -0.03 in.

At the nominal 78.81 in circumference, 1,000 rotations cover 2.00 km (1.24 mi); the inverse rates are 500/km and 804/mi.

Width endpoints in that set are 195/65 R15 (-1.18 in) and 255/35 R18 (+1.18 in) relative to 225/40 R18.

Curated alternatives: 12 sizes, spanning -1.3% to +1.5% in nominal diameter versus 225/40 R18.

Section-width mix versus 225/40 R18: 6 narrower, 3 equal-width and 3 wider entries.

This is a nominal geometry comparison only. Vehicle clearance, wheel width and offset, load and speed ratings, and manufacturer approval still require separate checks.

Specification

Specification
Overall diameter 25.09 in
Section width 8.86 in
Sidewall height 3.54 in
Circumference 78.81 in
Revolutions 500 /km · 804 /mi

In-tolerance sizes

Alternative sizes that stay within the selected diameter tolerance:

In-tolerance sizes
Size Diameter Speedo Verdict
255/35 R18 -0.2% -0.2% ✓ OK
195/65 R15 -0.4% -0.4% ✓ OK
225/35 R19 +0.5% +0.5% ✓ OK
225/45 R17 -0.5% -0.5% ✓ OK
195/60 R16 +0.5% +0.5% ✓ OK
205/55 R16 -0.8% -0.8% ✓ OK
215/55 R16 +0.9% +0.9% ✓ OK
225/50 R16 -0.9% -0.9% ✓ OK
235/45 R17 +1.0% +1.0% ✓ OK
235/40 R18 +1.3% +1.3% ✓ OK
195/70 R14 -1.3% -1.3% ✓ OK
215/50 R17 +1.5% +1.5% ✓ OK

Compare sizes

Looks valid
Looks valid
Units
Requires verificationWithin tolerance · Diameter -0.2% (-0.06 in) · US (~3%)
  • Requires verification · This rating covers tire geometry only, not vehicle compatibility.
  • Diameter -0.2% · Ride-height change -0.03 in.
  • Speedometer reads high · 60 mph → 59.9 mph.
  • Sidewall -0.03 in · Width +1.18 in.
Diameter-0.2%-0.06 in
Speedo @ 60-0.2%60 mph → 59.9 mph
Sidewall-0.03 inWidth +1.18 in
Ride-height change-0.03 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
25.09 in 25.03 in-0.06 in
Sidewall
3.54 in 3.51 in-0.03 in
Width
8.86 in 10.04 in+1.18 in
Circumference
78.81 in 78.63 in-0.2%
Revs / mile
804 806
Speedo @ 60
60 mph 59.9 mph-0.2%
Ride-height change
0.00 in -0.03 in
True distance per 100 indicated
100 mi 99.8 mi-0.2%
Δ⌀ -0.06 inΔ↔ +1.18 in
  • Current — 225/40 R18
  • New — 255/35 R18
  • Grounded to the road: the axle rises -0.03 in.

Result report

225/40 R18 255/35 R18 · Requires verification · Within tolerance

Plan budget for this size: 255/35 R18

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Plan budget for this size
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Budget$100–$140Per tireMid-range$130–$185Per tirePremium$185–$260Per tire

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

In-tolerance alternative sizes

In-tolerance alternative sizes
SizeDiameterSpeedo @ 100Use this
−1″ 205/50R17-0.1%-0.1%
OE 225/40R18+0.0%+0.0%

More tools

Last reviewed: 2026-07-22

Technical check
2026-07-22
Content updated
2026-07-22
What changed
  • Added a localized engine-derived geometry signature and matching Markdown narrative.
Methodology