Tyre Equivalent Size Calculator

Tyre Size Comparison Formula:

\[ \text{Overall Diameter} = \text{Rim Diameter} + 2 \times \left(\frac{\text{Section Width} \times \text{Aspect Ratio}}{100}\right) \]

Current Section Width:

Current Aspect Ratio:

Current Rim Diameter:

inches

New Section Width:

Current Overall Diameter:

Required Aspect Ratio for mm:

Definition: This calculator helps find equivalent tyre sizes when changing wheel/tyre dimensions while maintaining the same overall diameter.

Purpose: It ensures your speedometer remains accurate and prevents clearance issues when changing tyre sizes.

The calculator uses the formula:

\[ \text{Overall Diameter} = \text{Rim Diameter} + 2 \times \left(\frac{\text{Section Width} \times \text{Aspect Ratio}}{100}\right) \]

Where:

Details: Maintaining the same overall diameter ensures proper speedometer function, ABS operation, and prevents rubbing against wheel wells.

Tips: Enter your current tyre size (width/aspect/rim) and the new width you're considering to find the required aspect ratio.

Q1: How close should the new diameter match the original?
A: Ideally within ±3% of the original diameter to maintain proper vehicle operation.

Q2: Can I use this for plus-sizing wheels?
A: Yes, when increasing rim diameter, you'll need lower profile tyres to maintain overall diameter.

Q3: What if the exact aspect ratio isn't available?
A: Choose the closest available size, staying within the ±3% diameter tolerance.

Q4: Does this account for tyre pressure differences?
A: No, this calculates theoretical dimensions. Actual loaded diameters may vary slightly.

Q5: How does width change affect handling?
A: Wider tyres generally improve grip but may affect fuel economy and steering feel.