Automotive
Tire size
Calculate tire sidewall, overall diameter and circumference from metric size markings.
Calculators / Automotive / Tire size comparison
Tire size comparison calculator
Compare nominal tire diameter and the geometric change in speedometer reading.
On this page
Original nominal diameter631.9mm
New nominal diameter634.3mm
Diameter change0.38%
Estimated actual speed100.38km/h
Calculator controls
How to calculate
Compare nominal tire diameter and the geometric change in speedometer reading.
Diameter = rim inches × 25.4 + 2 × width mm × aspect ratio/100; actual speed = indicated speed × new diameter/original diameter.
Worked example
Example inputs
Original width (mm): 205 · Original aspect ratio (%): 55 · Original rim diameter (inches): 16 · New width (mm): 225 · New aspect ratio (%): 45 · New rim diameter (inches): 17 · Indicated speed (km/h): 100
Example resultOriginal nominal diameter: 631.9 mm · New nominal diameter: 634.3 mm · Diameter change: 0.37980693 % · Estimated actual speed: 100.37981 km/h
Assumptions and limits
Compare nominal tire diameter and the geometric change in speedometer reading. Assumes the original speedometer is accurate; loaded rolling radius, tire model and wear differ. This does not establish safe fitment, clearance or legal compliance.
Results are rounded for display; calculations use unrounded values. Read our calculation methodology.
Understanding the result
Compare nominal tire diameter and the geometric change in speedometer reading. Assumes the original speedometer is accurate; loaded rolling radius, tire model and wear differ. This does not establish safe fitment, clearance or legal compliance.
When this tool is useful
Diameter = rim inches × 25.4 + 2 × width mm × aspect ratio/100; actual speed = indicated speed × new diameter/original diameter.
Understanding your inputs
| Input | What to enter |
|---|---|
| Original width (mm) | Enter a number of at least 1 and no more than 1000. |
| Original aspect ratio (%) | Enter a number of at least 1 and no more than 200. |
| Original rim diameter (inches) | Enter a number of at least 1 and no more than 100. |
| New width (mm) | Enter a number of at least 1 and no more than 1000. |
| New aspect ratio (%) | Enter a number of at least 1 and no more than 200. |
| New rim diameter (inches) | Enter a number of at least 1 and no more than 100. |
| Indicated speed (km/h) | Enter a number of at least 0 and no more than 1000. |
Frequently asked questions
Does the diameter comparison confirm a tire fits safely?
No. It estimates nominal geometry and a speedometer difference. Check rim compatibility, actual tire dimensions, clearance at full steering and suspension travel, and vehicle approval.