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 in-tolerance plus/minus sizes you can switch to safely.
- Overall diameter: 29.61 in.
- Section width: 12.01 in.
- Sidewall height: 4.80 in.
- Revolutions: 423 /km · 681 /mi.
- In-tolerance sizes: 245/60 R18 (-0.1%).
Fitment verdict
245/60 R18 stays within the configured tolerance for 305/40 R20: -0.1% diameter and -0.1% speedometer effect.
| 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 |
Seasonal check
Planning a long summer drive?
Use the budget and running-cost tools before a trip, especially if the current tyres are worn or the replacement size changes diameter.
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 safe diameter tolerance:
| 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
- Within tolerance · Diameter -0.1%.
- Speedo @ 60: 60 mph → 59.9 mph.
- Sidewall +0.98 in · Width -2.36 in.
How this result is calculated
Computed from nominal tire-size geometry: diameter, circumference and speedometer error. It does not check wheel-arch, suspension or brake clearance.
Methodology- 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%
- 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 · Within tolerance
Use this size when shopping: 245/60 R18
Geometric estimate. Always verify physical clearance (arches, suspension, brakes) before fitting.
Safe alternative sizes
| Size | Diameter | Speedo @ 100 | Use this |
|---|---|---|---|
−1″ 245/55R19 | +0.0% | +0.0% | |
OE 305/40R20 | +0.0% | +0.0% | |
+1″ 145/75R21 | -0.1% | -0.1% | |
+2″ 215/45R22 | +0.0% | +0.0% |
More tools
- How to read a tire size
- Plus sizing guide
- Speedometer error guide
- 305mm Section width guide
- /40 Aspect ratio guide
- Rim diameter 20"
- Load index chart
- Speed rating chart
- Rim width and tyre fitment guide
- Wheel offset guide
- Wheel spacers guide
- Tyre pressure for heavy loads and towing
What changed
- Reviewed deterministic geometry, load/speed references, sitemap inclusion and localized page shell.