275/40R20 tyre diameter and speedometer calculator
A 275/40R20 tyre is 275 mm wide with a sidewall 40% of that — 110.0 mm — on a 20 inch rim. That gives an overall diameter of 728.0 mm.
- Overall diameter in inches
- 28.66
- Sidewall height
- 110.0
- Rolling circumference
- 2287.1
- Revolutions per kilometre
- 437.2
- Alternative size diameter
- 728.0
- Difference in diameter
- 0.000 % Within the usual 3% tolerance
- True speed on the alternative size
- 100.00
- Speedometer error
- 0.00
- Change in ride height
- 0.0
Show the working
- Alternative width = 275
- Alternative aspect ratio = 40
- Alternative rim = 20
- Indicated speed = 100
- width = 275 = 275
- aspect = 40 = 40
- rim = 20 = 20
- Sidewall height = 275 * 40 / 100 = 110
- Overall diameter = 20 * 25.4 + 2 * 110 = 728
- Rolling circumference = pi * 728 = 2287.08
- Revolutions per kilometre = 2287.08 > 0 ? 1e6 / 2287.08 : 0 = 437.239
- alt_sidewall = 275 * 40 / 100 = 110
- Alternative size diameter = 20 * 25.4 + 2 * 110 = 728
- Difference in diameter = 728 > 0 ? (728 - 728)/728*100 : 0 = 0
- True speed on the alternative size = 728 > 0 ? 100 * 728 / 728 : 0 = 100
- Speedometer error = 100 - 100 = 0
- Change in ride height = (728 - 728) / 2 = 0
- Overall diameter in inches = 728 / 25.4 = 28.6614
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- sidewall height = width x aspect ratio / 100
- overall diameter = rim diameter in mm + 2 x sidewall height
- true speed = indicated speed x new diameter / original diameter
Keep a replacement size within about 3% of the original diameter. Beyond that the speedometer, the odometer and often the stability control all read wrong.
The aspect ratio is a percentage of the section width, not a fixed height, which is why a wider tyre at the same aspect ratio is also taller.
Rolling circumference under load is slightly less than the free circumference because the tyre deflects. This is the unloaded figure.
Key dimensions of a 275/40R20
| Measurement | Metric | Imperial |
|---|---|---|
| Section width | 275 mm | 10.83 in |
| Sidewall height | 110.0 mm | 4.33 in |
| Rim diameter | 508.0 mm | 20 in |
| Overall diameter | 728.0 mm | 28.66 in |
| Circumference | 2287.1 mm | 90.0 in |
| Revolutions per km | 437 | 704 per mile |
Frequently asked
What is the overall diameter of a 275/40R20 tyre?
728.0 mm, or 28.66 inches.
What does 275/40R20 mean?
275 is the section width in millimetres, 40 is the sidewall height as a percentage of that width, R means radial construction, and 20 is the rim diameter in inches.