Tire makers publish measured revolutions per mile, which are a little higher than the calculated figure because a loaded tire is slightly squashed. Use the maker’s figure for gearing and speedometer calibration where you can.
How the tire size calculator works
For a metric size W/A R D, the sidewall height is W × A ÷ 100 millimetres and the overall diameter is D × 25.4 + 2 × sidewall. Circumference = π × diameter, revolutions per mile = 63,360 ÷ circumference in inches, and revolutions per km = 1,000,000 ÷ circumference in mm. With a new tire fitted, actual speed = speedometer reading × new diameter ÷ old diameter.
Common tire sizes in inches
| Size | Diameter | Width | Sidewall | Revs per mile |
|---|---|---|---|---|
| 195/65R15 | 24.98″ | 7.68″ | 4.99″ | 807 |
| 205/55R16 | 24.88″ | 8.07″ | 4.44″ | 811 |
| 215/55R17 | 26.31″ | 8.46″ | 4.66″ | 767 |
| 225/45R17 | 24.97″ | 8.86″ | 3.99″ | 808 |
| 225/65R17 | 28.52″ | 8.86″ | 5.76″ | 707 |
| 235/55R18 | 28.18″ | 9.25″ | 5.09″ | 716 |
| 245/40R18 | 25.72″ | 9.65″ | 3.86″ | 784 |
| 265/70R17 | 31.61″ | 10.43″ | 7.3″ | 638 |
| 275/55R20 | 31.91″ | 10.83″ | 5.95″ | 632 |
| 285/75R16 | 32.83″ | 11.22″ | 8.42″ | 614 |
TireSizeCalc calculates from the nominal size code (ISO / ETRTO metric and Tire and Rim Association P-metric and LT conventions). Actual dimensions differ by brand, tread depth, wheel width and load; manufacturers' spec sheets list the measured diameter and revolutions per mile. The ±3% guideline is a common rule of thumb, not a legal limit: check your vehicle's placard, load index, speed rating and clearance before changing size.
Frequently asked questions
How many revolutions per mile for a 225/45R17?
About 808 by calculation from the nominal size.
Why does revs per mile matter?
It sets speedometer and odometer calibration and the effective final drive ratio, which matters when changing tire size or axle gears.
Why is the published figure different?
Published figures are measured on a loaded, rolling tire, whose effective radius is smaller than the unloaded radius.
Is TireSizeCalc free?
Yes, with no sign-up or ads.
Are my sizes saved?
Yes, in this browser only.