255/40R19 tyre diameter and speedometer calculator
A 255/40R19 tyre is 255 mm wide with a sidewall 40% of that — 102.0 mm — on a 19 inch rim. That gives an overall diameter of 686.6 mm.
- Overall diameter in inches
- 27.03
- Sidewall height
- 102.0
- Rolling circumference
- 2157.0
- Revolutions per kilometre
- 463.6
- Alternative size diameter
- 686.6
- 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 = 255
- Alternative aspect ratio = 40
- Alternative rim = 19
- Indicated speed = 100
- width = 255 = 255
- aspect = 40 = 40
- rim = 19 = 19
- Sidewall height = 255 * 40 / 100 = 102
- Overall diameter = 19 * 25.4 + 2 * 102 = 686.6
- Rolling circumference = pi * 686.6 = 2157.02
- Revolutions per kilometre = 2157.02 > 0 ? 1e6 / 2157.02 : 0 = 463.603
- alt_sidewall = 255 * 40 / 100 = 102
- Alternative size diameter = 19 * 25.4 + 2 * 102 = 686.6
- Difference in diameter = 686.6 > 0 ? (686.6 - 686.6)/686.6*100 : 0 = 0
- True speed on the alternative size = 686.6 > 0 ? 100 * 686.6 / 686.6 : 0 = 100
- Speedometer error = 100 - 100 = 0
- Change in ride height = (686.6 - 686.6) / 2 = 0
- Overall diameter in inches = 686.6 / 25.4 = 27.0315
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
Scenarios & local history
Compare a saved scenario
Recent calculations
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 255/40R19
| Measurement | Metric | Imperial |
|---|---|---|
| Section width | 255 mm | 10.04 in |
| Sidewall height | 102.0 mm | 4.02 in |
| Rim diameter | 482.6 mm | 19 in |
| Overall diameter | 686.6 mm | 27.03 in |
| Circumference | 2157.0 mm | 84.9 in |
| Revolutions per km | 464 | 746 per mile |
Frequently asked
What is the overall diameter of a 255/40R19 tyre?
686.6 mm, or 27.03 inches.
What does 255/40R19 mean?
255 is the section width in millimetres, 40 is the sidewall height as a percentage of that width, R means radial construction, and 19 is the rim diameter in inches.