TireFitLab

305/40 R20

What size is 305/40 R20?

305/40 R20 has an overall diameter of about 29.61 in, a section width of 12.01 in and a sidewall height of 4.80 in. It turns roughly 681 times per mile. Below are the exact figures plus plus/minus sizes that stay within the selected diameter tolerance.

Geometric comparison

245/60 R18 stays within the configured tolerance for 305/40 R20: -0.1% diameter and -0.1% speedometer effect.

Geometric comparison
Metric This size: 305/40 R20 Alternative: 245/60 R18 Difference Verdict
Overall diameter 29.61 in 29.57 in -0.1% Within tolerance
Section width 12.01 in 9.65 in -2.36 in Computed difference
Sidewall height 4.80 in 5.79 in +0.98 in Computed difference
Circumference 93.01 in 92.91 in -0.1% Within tolerance
Speedo 0.0% -0.1% -0.1% Within tolerance

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What this geometry means

305/40 R20 has a low-profile proportion. The marking combines nominal section width 12.01 in, sidewall ratio 40% and rim diameter 20 inches. One sidewall is 4.80 in; both sidewalls make up 32% of the 29.61 in overall diameter, while the rim makes up 68%.

The closest diameter entry is 245/60 R18 at -0.1%. Relative to 305/40 R20, it changes nominal section width by -2.36 in, each sidewall by +0.98 in and radius by -0.02 in.

At the nominal 93.01 in circumference, 1,000 rotations cover 2.36 km (1.47 mi); the inverse rates are 423/km and 681/mi.

Width endpoints in that set are 235/70 R17 (-2.76 in) and 295/40 R20 (-0.39 in) relative to 305/40 R20.

Curated alternatives: 12 sizes, spanning -1.4% to +1.5% in nominal diameter versus 305/40 R20.

Section-width mix versus 305/40 R20: 12 narrower, 0 equal-width and 0 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 29.61 in
Section width 12.01 in
Sidewall height 4.80 in
Circumference 93.01 in
Revolutions 423 /km · 681 /mi

In-tolerance sizes

Alternative sizes that stay within the selected diameter tolerance:

In-tolerance sizes
Size Diameter Speedo Verdict
245/60 R18 -0.1% -0.1% ✓ OK
245/65 R17 -0.2% -0.2% ✓ OK
245/70 R16 -0.3% -0.3% ✓ OK
275/45 R20 +0.5% +0.5% ✓ OK
265/50 R19 -0.6% -0.6% ✓ OK
265/45 R20 -0.7% -0.7% ✓ OK
285/35 R22 +0.8% +0.8% ✓ OK
295/40 R20 -1.1% -1.1% ✓ OK
235/70 R17 +1.2% +1.2% ✓ OK
235/65 R18 +1.4% +1.4% ✓ OK
235/55 R19 -1.4% -1.4% ✓ OK
255/50 R20 +1.5% +1.5% ✓ OK

Compare sizes

Looks valid
Looks valid
Units
Not recommendedWithin tolerance · Diameter -0.1% (-0.03 in) · US (~3%)
  • Not recommended · This rating covers tire geometry only, not vehicle compatibility.
  • Diameter -0.1% · Ride-height change -0.02 in.
  • Speedometer reads high · 60 mph → 59.9 mph.
  • Sidewall +0.98 in · Width -2.36 in.
Diameter-0.1%-0.03 in
Speedo @ 60-0.1%60 mph → 59.9 mph
Sidewall+0.98 inWidth -2.36 in
Ride-height change-0.02 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
29.61 in 29.57 in-0.03 in
Sidewall
4.80 in 5.79 in+0.98 in
Width
12.01 in 9.65 in-2.36 in
Circumference
93.01 in 92.91 in-0.1%
Revs / mile
681 682
Speedo @ 60
60 mph 59.9 mph-0.1%
Ride-height change
0.00 in -0.02 in
True distance per 100 indicated
100 mi 99.9 mi-0.1%
Δ⌀ -0.03 inΔ↔ -2.36 in
  • Current — 305/40 R20
  • New — 245/60 R18
  • Grounded to the road: the axle rises -0.02 in.

Result report

305/40 R20 245/60 R18 · Not recommended · 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 305/40R20+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