How to Use Tire Size Calculator
The Tire Size Calculator makes it easy to decode any standard tire specification and compare two tire sizes side by side.
- Enter Original Tire — Input the width (mm), aspect ratio (%), and rim diameter (inches) of your current tire, e.g. 225 / 45 / 17.
- Enter New Tire — Fill in the same three fields for the replacement tire you are considering.
- Read the Results — The Tire Size Calculator instantly shows sidewall height, total diameter, rolling circumference, revolutions per km, and the speedometer error between the two sizes.
A green badge means the size difference is within the safe ±3% threshold. A red badge warns you that the speedometer deviation may be significant enough to affect legality or safety.
Formula & Theory — Tire Size Calculator
The Tire Size Calculator uses standard automotive engineering formulas:
Sidewall Height (mm) = Tire Width × (Aspect Ratio / 100)
Total Diameter (mm) = Rim Diameter × 25.4 + 2 × Sidewall Height
Circumference (mm) = π × Total Diameter
Revolutions per km = 1,000,000 / Circumference
Speedometer Error (%) = (New Circumference − Original Circumference) / Original Circumference × 100
| Symbol | Meaning |
|---|---|
| Width | Section width of the tire in millimeters |
| Aspect Ratio | Sidewall height as a percentage of tire width |
| Rim Diameter | Wheel diameter in inches |
A larger overall diameter means the vehicle travels farther per revolution, causing the speedometer to under-read actual speed. A smaller diameter causes over-reading. The Tire Size Calculator quantifies this difference so you can choose a compatible replacement.
Acceptable Size Deviation
The general guideline accepted by most mechanics and tire specialists is a speedometer error of no more than ±3%. Exceeding this threshold may affect odometer accuracy, ABS calibration, and in some regions, legal compliance.
Use Cases for Tire Size Calculator
The Tire Size Calculator is a practical tool for a wide range of situations:
- Wheel Upgrades — Check whether plus-sizing (larger rim, lower profile) keeps the rolling diameter close enough to stock.
- Winter / Summer Tire Swaps — Verify that a seasonal set purchased online matches the total diameter of your OEM tires.
- Used Car Inspection — Quickly confirm that the tires currently fitted match the manufacturer's recommended size.
- Cross-Border Purchases — Compare tire sizes listed in different regional formats before ordering from overseas retailers.
Use the Tire Size Calculator before every tire purchase to avoid unexpected handling changes, speedometer drift, or clearance issues inside the wheel arch.