1. What is a Tyre Size Calculator for Australia?

Definition: This calculator determines the overall diameter of a tyre based on rim size and sidewall height, using Australian standard measurements.

Purpose: It helps vehicle owners, mechanics, and tyre professionals calculate exact tyre dimensions for proper fitment and compliance.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• Diameter — Total tyre diameter in millimeters
• Rim — Rim diameter in inches
• Sidewall — Sidewall height in millimeters

Explanation: The rim diameter is converted from inches to mm (×25.4) and added to twice the sidewall height (since sidewall appears on both sides of the rim).

3. Importance of Accurate Tyre Diameter Calculation

Details: Correct tyre diameter ensures proper speedometer accuracy, vehicle clearance, suspension geometry, and compliance with Australian road regulations.

4. Using the Calculator

Tips: Enter the rim size in inches (e.g., 16 for 16-inch rims) and sidewall height in millimeters (as shown on tyre sidewall). All values must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: Where do I find the sidewall height on my tyre?
A: Look for numbers like "225/45R17" - the middle number (45) is the aspect ratio percentage of the width.

Q2: How do I calculate sidewall height from aspect ratio?
A: Multiply tyre width (first number) by aspect ratio percentage (e.g., 225mm × 45% = 101.25mm sidewall).

Q3: Why is tyre diameter important?
A: Affects speedometer accuracy, gear ratios, ABS operation, and may be legally regulated in Australia.

Q4: What's the 25.4 conversion factor?
A: This converts inches to millimeters (1 inch = 25.4mm) for consistent metric measurements.

Q5: How precise should these measurements be?
A: For safety and compliance, measurements should be accurate to within ±5mm of actual diameter.