Expert Reviewed by: David Chen, Automotive Engineer & Wheel Fitment Specialist
The Tyre Stretch Calculator helps automotive enthusiasts and tuners determine the fitment compatibility between specific tyre widths and rim widths. Use this tool to visualize how much a tyre will “stretch” across a wider wheel, ensuring both style and safety considerations are met.
Tyre Stretch Calculator
Tyre Stretch Calculator Formula
Stretch Factor (SF) = (Rim Width in mm) / Tyre Width
Rim Width (mm) = Rim Width (inches) × 25.4
Sidewall Height = Tyre Width × (Aspect Ratio / 100)
Source: Wheel-Size.com Technical Reference and WillTheyFit.com Fitment Guide.
Variables:
- Tyre Width: The width of the tyre from sidewall to sidewall in millimeters.
- Aspect Ratio: The height of the sidewall as a percentage of the tyre width.
- Rim Width: The distance between the beads of the wheel in inches.
What is a Tyre Stretch Calculator?
A tyre stretch calculator is a specialized tool used primarily in the “stanced” car community. It determines how a tyre of a specific size will sit on a wheel that is wider than the manufacturer’s standard recommendation. This “stretch” pulls the sidewalls inward, creating a distinct aesthetic.
Beyond aesthetics, this calculator helps you understand the mechanical deviation from standard fitment. Extreme stretching can affect bead seating, air pressure retention, and overall handling characteristics.
How to Calculate Tyre Stretch (Example)
- Identify Inputs: Let’s use a 225/40 tyre on a 9.5-inch rim.
- Convert Rim Width: 9.5 inches × 25.4 = 241.3 mm.
- Compare Widths: 241.3 mm (Rim) vs 225 mm (Tyre). The rim is 16.3 mm wider than the tyre.
- Calculate Ratio: 241.3 / 225 = 1.07. This indicates a “Moderate” stretch profile.
Related Calculators
- Wheel Offset Calculator
- Speedometer Error Calculator
- Tyre Sidewall Height Tool
- Bolt Pattern Cross Reference
Frequently Asked Questions (FAQ)
Is tyre stretching safe? Moderate stretching is common in tuning, but extreme stretch can increase the risk of “de-beading” or sidewall damage under heavy cornering.
How does stretch affect my speedometer? Stretching a tyre slightly reduces its rolling diameter, which can cause your speedometer to read slightly faster than your actual speed.
What is a “square” fitment? This is when the tyre width and rim width are perfectly matched so the sidewall stands vertical without leaning in or out.
Do I need higher PSI for stretched tyres? Yes, usually. Stretched tyres require higher air pressure (often 38-45 PSI) to ensure the bead stays seated firmly against the rim lip.