TireFitLab

255/45 R18

What size is 255/45 R18?

255/45 R18 has an overall diameter of about 27.04 in, a section width of 10.04 in and a sidewall height of 4.52 in. It turns roughly 746 times per mile. Below are the exact figures plus plus/minus sizes that stay within the selected diameter tolerance.

Geometric comparison

255/40 R19 stays within the configured tolerance for 255/45 R18: 0.0% diameter and 0.0% speedometer effect.

Geometric comparison
Metric This size: 255/45 R18 Alternative: 255/40 R19 Difference Verdict
Overall diameter 27.04 in 27.03 in 0.0% Within tolerance
Section width 10.04 in 10.04 in 0.00 in Computed difference
Sidewall height 4.52 in 4.02 in -0.50 in Computed difference
Circumference 84.93 in 84.92 in 0.0% Within tolerance
Speedo 0.0% 0.0% 0.0% Within tolerance

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What this geometry means

255/45 R18 has a mid-range sidewall proportion. The marking combines nominal section width 10.04 in, sidewall ratio 45% and rim diameter 18 inches. One sidewall is 4.52 in; both sidewalls make up 33% of the 27.04 in overall diameter, while the rim makes up 67%.

At the nominal 84.93 in circumference, 1,000 rotations cover 2.16 km (1.34 mi); the inverse rates are 464/km and 746/mi.

Section-width mix versus 255/45 R18: 10 narrower, 2 equal-width and 0 wider entries.

The closest diameter entry is 255/40 R19 at -0.0%. Relative to 255/45 R18, it changes nominal section width by +0.00 in, each sidewall by -0.50 in and radius by -0.00 in.

Curated alternatives: 12 sizes, spanning -1.3% to +0.8% in nominal diameter versus 255/45 R18.

Width endpoints in that set are 215/65 R16 (-1.57 in) and 255/35 R20 (+0.00 in) relative to 255/45 R18.

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 27.04 in
Section width 10.04 in
Sidewall height 4.52 in
Circumference 84.93 in
Revolutions 464 /km · 746 /mi

In-tolerance sizes

Alternative sizes that stay within the selected diameter tolerance:

In-tolerance sizes
Size Diameter Speedo Verdict
255/40 R19 -0.0% -0.0% ✓ OK
255/35 R20 -0.0% -0.0% ✓ OK
215/65 R16 -0.1% -0.1% ✓ OK
235/60 R16 +0.2% +0.2% ✓ OK
215/60 R17 +0.5% +0.5% ✓ OK
235/55 R17 +0.5% +0.5% ✓ OK
215/70 R15 -0.7% -0.7% ✓ OK
235/50 R18 +0.8% +0.8% ✓ OK
245/35 R20 -1.0% -1.0% ✓ OK
225/55 R17 -1.1% -1.1% ✓ OK
245/40 R19 -1.2% -1.2% ✓ OK
245/45 R18 -1.3% -1.3% ✓ OK

Compare sizes

Looks valid
Looks valid
Units
Requires verificationWithin tolerance · Diameter -0.0% (-0.00 in) · US (~3%)
  • Requires verification · This rating covers tire geometry only, not vehicle compatibility.
  • Diameter -0.0% · Ride-height change -0.00 in.
  • Speedometer unchanged · 60 mph → 60.0 mph.
  • Sidewall -0.50 in · Width +0.00 in.
Diameter-0.0%-0.00 in
Speedo @ 60-0.0%60 mph → 60.0 mph
Sidewall-0.50 inWidth +0.00 in
Ride-height change-0.00 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
27.04 in 27.03 in-0.00 in
Sidewall
4.52 in 4.02 in-0.50 in
Width
10.04 in 10.04 in+0.00 in
Circumference
84.93 in 84.92 in-0.0%
Revs / mile
746 746
Speedo @ 60
60 mph 60.0 mph-0.0%
Ride-height change
0.00 in -0.00 in
True distance per 100 indicated
100 mi 100.0 mi-0.0%
Δ⌀ -0.00 inΔ↔ +0.00 in
  • Current — 255/45 R18
  • New — 255/40 R19
  • Grounded to the road: the axle rises -0.00 in.

Result report

255/45 R18 255/40 R19 · Requires verification · Within tolerance

Plan budget for this size: 255/40 R19

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Plan budget for this size
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Budget$125–$175Per tireMid-range$160–$230Per tirePremium$230–$325Per tire

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

In-tolerance alternative sizes

In-tolerance alternative sizes
SizeDiameterSpeedo @ 100Use this
−1″ 255/50R17+0.0%+0.0%
OE 255/45R18+0.0%+0.0%
+1″ 255/40R19-0.0%-0.0%

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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