What this tool does
Translate tire markings into physical dimensions and see how a proposed size changes rolling geometry.
How to use it
- Enter width, aspect ratio and rim size for the current tire.
- Enter the proposed size and an indicated speed.
- Compare diameter, circumference, ride height and estimated actual speed.
Formula and method
Sidewall = width × aspect ratio. Diameter = rim inches × 25.4 + two sidewalls; circumference = π × diameter. Speed scales by replacement diameter ÷ original diameter.
Example
Changing 205/55R16 to 225/45R17 increases diameter by about 2.4 mm, raises the vehicle about 1.2 mm and makes actual speed slightly higher than indicated.
Useful for
- Comparing common replacement sizes
- Estimating speedometer direction
- Checking geometry before professional fitment
Limits and notes
The result does not confirm clearance, rim-width compatibility, load index, speed rating, pressure, suspension, braking, legal approval or manufacturer warranty. A qualified tire professional must approve fitment.
Frequently asked questions
Is a small diameter difference automatically safe?
No. Diameter is only one factor; clearance, load, rim and vehicle requirements still apply.
Why is actual speed different?
A larger rolling circumference travels farther per wheel rotation than the original calibration.