Tire Size Calculator
Adjust the inputs below. Results update as you type.
How it works
Format P[width]/[aspect ratio]R[wheel diameter]: 225/65R17 means 225mm wide, 65% aspect ratio, 17-inch rim. Sidewall height = width × aspect ratio / 100 = 225 × 0.65 = 146.25mm. Overall diameter = wheel diameter × 25.4 + 2 × sidewall = 431.8 + 292.5 = 724.3mm. Changing tire size affects speedometer accuracy and gearing. A 3% diameter change alters speedometer reading by 3%.
Input guidance
- Confirm date/time/unit settings before comparing outputs.
- If a task has optional fields, run both with and without them to understand impact.
- Save scenario variants when making practical decisions.
The formula
Tire size like 225/45R17 means 225 mm width, 45% aspect ratio (sidewall height as a percent of width), and a 17-inch wheel. Overall diameter = wheel + 2 × sidewall.
Worked example
A 225/45R17: sidewall = 225 × 0.45 = 101 mm; diameter = 17 in (432 mm) + 2 × 101 = 634 mm, about 25 inches.
More examples to test
- Quick-pass example: run default assumptions for a first estimate.
- Refined example: update one assumption at a time to isolate impact.
How to interpret results
Use outputs as operational estimates and pair them with local constraints, business rules, or provider requirements.
When this can be inaccurate
Utilities can miss local policy details, special-case rules, and environment-specific constraints.
Change history
- July 2026: Quality-reviewed for publication with formula checks and explanatory copy updates.
Site-wide corrections also appear on the corrections log.
Find productivity tools for everyday workflows
General recommendation, not professional advice.
Frequently asked questions
What do the numbers on a tire mean?+
Width in millimeters, aspect ratio (sidewall as a percent of width), and wheel diameter in inches, e.g. 205/55R16.
Why does changing tire size affect my speedometer?+
A different overall diameter changes how far the car travels per revolution, so the speedometer reads high or low unless recalibrated.
Can I fit a different size?+
Plus-sizing keeps overall diameter similar while changing width or wheel size. Large deviations affect handling, clearance, and accuracy.
Why are outputs slightly different from another tool?+
Different tools can use different rounding rules, default assumptions, and treatment of edge cases.
How can I improve estimate reliability?+
Use verified inputs from real records and re-run calculations after major assumption changes.