CalcCommonsHow to use
Read the width, aspect ratio and rim diameter from each tire sidewall. Enter the original size first because speedometer comparison uses it as the reference. Select the speed unit and compare dimensions before consulting your vehicle and tire specifications.
For related tasks, try the Percentage Calculator.
Formula & how it works
Sidewall height = width in mm × aspect ratio / 100. Diameter in mm = rim inches × 25.4 + 2 × sidewall. Circumference = π × diameter. Actual speed = indicated speed × new diameter / original diameter.
Worked examples
Example 1
For 225/60R17, sidewall height is 225 × 0.60 = 135 mm. Diameter is 17 × 25.4 + 270 = 701.8 mm, or about 27.63 inches.
Example 2
Changing from 225/60R17 to 235/55R18 changes nominal diameter from 701.8 to 715.7 mm, an increase of about 1.98%. An indicated 60 mph corresponds geometrically to about 61.19 mph. Ride-height change is half the diameter difference: 6.95 mm.
Assumptions & practical notes
This calculator accepts metric section-width sizes such as 225/60R17. It does not decode flotation sizes such as 33×12.50R17.
Nominal diameter is not measured loaded rolling circumference. Tire construction, pressure, tread wear and load influence actual values.
A small diameter difference does not establish fitment or safety. Check manufacturer-approved sizes, load and speed ratings, rim widths, brake clearance and vehicle electronics.
See our calculation methodology for unit definitions and testing practices.
References & measurement guidance
References explain the underlying mathematics. The examples and interactive calculations on this page are specific to CalcCommons.
Frequently asked questions
What does 225/60R17 mean?
225 is nominal section width in millimeters; 60 is sidewall height as a percentage of that width; R indicates radial construction; 17 is wheel diameter in inches.
Will a larger tire change my speedometer?
If calibration remains unchanged, a larger rolling circumference travels farther per revolution. This tool estimates the effect using nominal diameter ratios.
Does this tell me whether a tire will fit?
No. Diameter alone does not establish clearance, rim compatibility, load capacity or manufacturer approval.
Why is ground clearance half the diameter change?
The axle center moves by the radius difference in the ideal unloaded geometry, which is half the diameter difference. Real loaded clearance can differ.