TireFitLab

245/40 R18

What size is 245/40 R18?

245/40 R18 has an overall diameter of about 25.72 in, a section width of 9.65 in and a sidewall height of 3.86 in. It turns roughly 784 times per mile. Below are the exact figures plus plus/minus sizes that stay within the selected diameter tolerance.

Geometric comparison

225/55 R16 stays within the configured tolerance for 245/40 R18: +0.1% diameter and +0.1% speedometer effect.

Geometric comparison
Metric This size: 245/40 R18 Alternative: 225/55 R16 Difference Verdict
Overall diameter 25.72 in 25.74 in +0.1% Within tolerance
Section width 9.65 in 8.86 in -0.79 in Computed difference
Sidewall height 3.86 in 4.87 in +1.01 in Computed difference
Circumference 80.79 in 80.88 in +0.1% Within tolerance
Speedo 0.0% +0.1% +0.1% Within tolerance

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What this geometry means

245/40 R18 has a low-profile proportion. The marking combines nominal section width 9.65 in, sidewall ratio 40% and rim diameter 18 inches. One sidewall is 3.86 in; both sidewalls make up 30% of the 25.72 in overall diameter, while the rim makes up 70%.

The closest diameter entry is 225/55 R16 at +0.1%. Relative to 245/40 R18, it changes nominal section width by -0.79 in, each sidewall by +1.01 in and radius by +0.01 in.

At the nominal 80.79 in circumference, 1,000 rotations cover 2.05 km (1.28 mi); the inverse rates are 487/km and 784/mi.

Width endpoints in that set are 205/60 R16 (-1.57 in) and 255/35 R19 (+0.39 in) relative to 245/40 R18.

Curated alternatives: 12 sizes, spanning -1.2% to +1.2% in nominal diameter versus 245/40 R18.

Section-width mix versus 245/40 R18: 9 narrower, 2 equal-width and 1 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 25.72 in
Section width 9.65 in
Sidewall height 3.86 in
Circumference 80.79 in
Revolutions 487 /km · 784 /mi

In-tolerance sizes

Alternative sizes that stay within the selected diameter tolerance:

In-tolerance sizes
Size Diameter Speedo Verdict
225/55 R16 +0.1% +0.1% ✓ OK
205/60 R16 -0.1% -0.1% ✓ OK
245/35 R19 +0.1% +0.1% ✓ OK
245/45 R17 -0.1% -0.1% ✓ OK
225/50 R17 +0.6% +0.6% ✓ OK
205/55 R17 +0.6% +0.6% ✓ OK
205/65 R15 -0.9% -0.9% ✓ OK
235/35 R19 -0.9% -0.9% ✓ OK
215/50 R17 -1.0% -1.0% ✓ OK
225/45 R18 +1.0% +1.0% ✓ OK
255/35 R19 +1.2% +1.2% ✓ OK
235/40 R18 -1.2% -1.2% ✓ OK

Compare sizes

Looks valid
Looks valid
Units
High fitment riskWithin tolerance · Diameter +0.1% (+0.03 in) · US (~3%)
  • High fitment risk · This rating covers tire geometry only, not vehicle compatibility.
  • Diameter +0.1% · Ride-height change +0.01 in.
  • Speedometer reads low · 60 mph → 60.1 mph.
  • Sidewall +1.01 in · Width -0.79 in.
Diameter+0.1%+0.03 in
Speedo @ 60+0.1%60 mph → 60.1 mph
Sidewall+1.01 inWidth -0.79 in
Ride-height change+0.01 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
25.72 in 25.74 in+0.03 in
Sidewall
3.86 in 4.87 in+1.01 in
Width
9.65 in 8.86 in-0.79 in
Circumference
80.79 in 80.88 in+0.1%
Revs / mile
784 783
Speedo @ 60
60 mph 60.1 mph+0.1%
Ride-height change
0.00 in +0.01 in
True distance per 100 indicated
100 mi 100.1 mi+0.1%
Δ⌀ +0.03 inΔ↔ -0.79 in
  • Current — 245/40 R18
  • New — 225/55 R16
  • Grounded to the road: the axle rises +0.01 in.

Result report

245/40 R18 225/55 R16 · 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
OE 245/40R18+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