275/40 R20 vs 255/55 R18

What changes when you change tire size?

255/55 R18 stays within the configured tolerance for 275/40 R20: +1.3% diameter and +1.3% speedometer effect.

Geometric comparison

Geometric comparison
Metric This size: 275/40 R20 Alternative: 255/55 R18 Difference Verdict
Overall diameter 28.66 in 29.04 in +1.3% Within tolerance
Section width 10.83 in 10.04 in -0.79 in Computed difference
Sidewall height 4.33 in 5.52 in +1.19 in Computed difference
Circumference 90.04 in 91.24 in +1.3% Within tolerance
Speedo 0.0% +1.3% +1.3% Within tolerance

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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.

High fitment riskWithin tolerance · Diameter +1.3% (+0.38 in) · US preset (±3%)
  • High fitment risk · This rating covers tire geometry only, not vehicle compatibility.
  • Diameter +1.3% · Ride-height change +0.19 in.
  • Speedometer reads low · 60 mph → 60.8 mph.
  • Sidewall +1.19 in · Width -0.79 in.
Diameter+1.3%+0.38 in
Speedo @ 60+1.3%60 mph → 60.8 mph
Sidewall+1.19 inWidth -0.79 in
Ride-height change+0.19 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
28.66 in 29.04 in+0.38 in
Sidewall
4.33 in 5.52 in+1.19 in
Width
10.83 in 10.04 in-0.79 in
Circumference
90.04 in 91.24 in+1.3%
Revs / mile
704 694
Speedo @ 60
60 mph 60.8 mph+1.3%
Ride-height change
0.00 in +0.19 in
True distance per 100 indicated
100 mi 101.3 mi+1.3%
Δ⌀ +0.38 inΔ↔ -0.79 in
  • Current — 275/40 R20
  • New — 255/55 R18
  • Grounded to the road: the axle rises +0.19 in.

Tires and wheels in one comparison

Result report

275/40 R20 255/55 R18 · 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″ 265/45R19-0.9%-0.9%
OE 275/40R20+0.0%+0.0%
+1″ 285/35R21+0.7%+0.7%
+2″ 285/30R22+0.2%+0.2%

275/40 R20 · 255/55 R18 · Methodology