1. What is a Tyre Size Speed Calculator?

Definition: This calculator determines how changing your tyre size affects your vehicle's speedometer reading and actual speed.

Purpose: It helps Australian drivers understand the impact of tyre size changes on their vehicle's speed measurement and performance.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• New Speed — Actual vehicle speed with new tyres (km/h)
• Old Diameter — Original tyre diameter (mm)
• New Diameter — Changed tyre diameter (mm)
• Original Speed — Speedometer reading (km/h)

Explanation: Larger tyres make your speedometer read slower than actual speed, while smaller tyres make it read faster.

3. Importance of Tyre Size Calculations

Details: Correct calculations help maintain accurate speed readings, ensure legal compliance, and prevent odometer discrepancies.

4. Using the Calculator

Tips: Enter your original tyre diameter, new tyre diameter, and a reference speed (default 100 km/h). All values must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: How do I find my tyre diameter?
A: Use the formula: Diameter (mm) = (Width × Aspect Ratio × 2 / 100) + (Rim Diameter × 25.4)

Q2: Is this calculator specific to Australia?
A: While the formula is universal, it's particularly relevant for Australia where changing tyre sizes is common for 4WDs.

Q3: What's the legal tolerance for speedometer accuracy?
A: In Australia, speedometers must not under-read and can over-read by up to 10% + 4 km/h.

Q4: How does this affect my odometer?
A: Larger tyres cause your odometer to record fewer kilometers than actually traveled, while smaller tyres record more.

Q5: Should I get my speedometer recalibrated?
A: If your speed difference exceeds 3-5%, consider professional recalibration to maintain accuracy.