How it’s calculated
- Sidewall (in) = width (mm) × aspect ÷ 100 ÷ 25.4
- Diameter = wheel + 2 × sidewall
- Revolutions per mile = 63,360 ÷ (π × diameter)
- Actual speed = indicated × new diameter ÷ old diameter
Worked example
Changing from 265/70R17 to 275/65R18.
Inputs
- Tire width
- 265 mm
- Aspect ratio
- 70 %
- Wheel diameter
- 17 in
- Compare with a new size
- Yes
- New tire width
- 275 mm
- New aspect ratio
- 65 %
- New wheel diameter
- 18 in
- Indicated speed
- 60 mph
Results
- Overall diameter
- 31.61 in
- Sidewall height
- 7.3 in
- Circumference
- 99.29 in
- Revolutions per mile
- 638.1
- New diameter
- 32.07 in
- Diameter difference
- 1.48%
- Actual speed at indicated speed
- 60.9 mph
Assumptions and limits
- Nominal unloaded dimensions. Real tires vary by model and inflation, and loaded radius is smaller.
Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.
Questions
How tall is a 265/70R17 tire?
About 31.6 in in diameter, with a 7.3 in sidewall.
Will bigger tires change my speedometer?
Yes. A larger diameter covers more ground per revolution, so you travel faster than indicated.