The odometer is off by the same percentage. Many vehicles can be recalibrated by a dealer or with a programmer.
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
Do bigger tires make you go faster than the speedometer says?
Yes. A tire 3% taller means 61.8 mph when the speedometer shows 60.
How do I correct the speedometer?
Many modern vehicles can be reprogrammed with the new tire size by a dealer or a tuner; older ones may need a different drive gear.
Does it affect the odometer?
Yes, by the same percentage as the speedometer.
Is TireSizeCalc free?
Yes, with no sign-up or ads.
Are my sizes saved?
Yes, in this browser only.