Tyre Fitment Calculator

Tyre Fitment Calculator

Enter your current and desired tyre specifications to compare their dimensions and understand the impact on your vehicle's speedometer.

Original Tyre Specifications




New Tyre Specifications




Calculation Results:

Enter values and click 'Calculate' to see results.

function calculateTyreFitment() { var originalTyreWidth = parseFloat(document.getElementById('originalTyreWidth').value); var originalAspectRatio = parseFloat(document.getElementById('originalAspectRatio').value); var originalWheelDiameter = parseFloat(document.getElementById('originalWheelDiameter').value); var newTyreWidth = parseFloat(document.getElementById('newTyreWidth').value); var newAspectRatio = parseFloat(document.getElementById('newAspectRatio').value); var newWheelDiameter = parseFloat(document.getElementById('newWheelDiameter').value); var resultDiv = document.getElementById('tyreFitmentResult'); if (isNaN(originalTyreWidth) || isNaN(originalAspectRatio) || isNaN(originalWheelDiameter) || isNaN(newTyreWidth) || isNaN(newAspectRatio) || isNaN(newWheelDiameter) || originalTyreWidth <= 0 || originalAspectRatio <= 0 || originalWheelDiameter <= 0 || newTyreWidth <= 0 || newAspectRatio <= 0 || newWheelDiameter <= 0) { resultDiv.innerHTML = "Please enter valid positive numbers for all tyre specifications."; return; } // Constants var INCH_TO_MM = 25.4; var PI = Math.PI; // Calculate Original Tyre Specs var originalSidewallHeight = originalTyreWidth * (originalAspectRatio / 100); var originalOverallDiameter = (2 * originalSidewallHeight) + (originalWheelDiameter * INCH_TO_MM); var originalCircumference = originalOverallDiameter * PI; // Calculate New Tyre Specs var newSidewallHeight = newTyreWidth * (newAspectRatio / 100); var newOverallDiameter = (2 * newSidewallHeight) + (newWheelDiameter * INCH_TO_MM); var newCircumference = newOverallDiameter * PI; // Calculate Differences var diameterDifference = newOverallDiameter – originalOverallDiameter; var diameterPercentageChange = (diameterDifference / originalOverallDiameter) * 100; var circumferenceDifference = newCircumference – originalCircumference; var circumferencePercentageChange = (circumferenceDifference / originalCircumference) * 100; // Speedometer Error Calculation // If new tyre is larger, speedometer reads lower than actual speed. // If new tyre is smaller, speedometer reads higher than actual speed. var speedometerErrorPercentage = diameterPercentageChange; // Same as diameter percentage change var speedoReading = 60; // Example speed in mph/kmh var actualSpeed = speedoReading * (1 + (speedometerErrorPercentage / 100)); var resultHTML = "

Original Tyre:

"; resultHTML += "Sidewall Height: " + originalSidewallHeight.toFixed(2) + " mm"; resultHTML += "Overall Diameter: " + originalOverallDiameter.toFixed(2) + " mm"; resultHTML += "Circumference: " + originalCircumference.toFixed(2) + " mm"; resultHTML += "

New Tyre:

"; resultHTML += "Sidewall Height: " + newSidewallHeight.toFixed(2) + " mm"; resultHTML += "Overall Diameter: " + newOverallDiameter.toFixed(2) + " mm"; resultHTML += "Circunference: " + newCircumference.toFixed(2) + " mm"; resultHTML += "

Comparison:

"; resultHTML += "Diameter Change: " + diameterDifference.toFixed(2) + " mm (" + diameterPercentageChange.toFixed(2) + "%)"; resultHTML += "Circumference Change: " + circumferenceDifference.toFixed(2) + " mm (" + circumferencePercentageChange.toFixed(2) + "%)"; resultHTML += "Speedometer Error: " + speedometerErrorPercentage.toFixed(2) + "%"; if (Math.abs(speedometerErrorPercentage) > 0.1) { // Only show actual speed if there's a noticeable difference var errorDirection = speedometerErrorPercentage > 0 ? "lower" : "higher"; resultHTML += "When your speedometer reads " + speedoReading + " mph (or km/h), your actual speed will be approximately " + actualSpeed.toFixed(2) + " mph (or km/h)."; if (Math.abs(speedometerErrorPercentage) > 3) { resultHTML += "Warning: A speedometer error of " + speedometerErrorPercentage.toFixed(2) + "% is significant and may affect safety and legality. Consider a different tyre size."; } } else { resultHTML += "The change in tyre size is minimal and will have a negligible effect on your speedometer reading."; } resultDiv.innerHTML = resultHTML; }

Understanding Tyre Fitment and Why It Matters

Changing your vehicle's tyres might seem straightforward, but selecting the correct size is crucial for safety, performance, and maintaining the accuracy of your speedometer. A tyre fitment calculator helps you compare different tyre sizes and understand the implications of switching from one size to another.

How Tyre Sizes Are Designated

Tyre sizes are typically displayed as a series of numbers and letters, for example, 205/55R16. Let's break down what each part means:

  • 205: This is the tyre width in millimeters (mm). It measures the width of the tyre from sidewall to sidewall.
  • 55: This is the aspect ratio, expressed as a percentage. It represents the height of the tyre's sidewall as a percentage of its width. In this example, the sidewall height is 55% of 205mm.
  • R: This indicates the tyre's construction type, with 'R' standing for Radial, the most common type.
  • 16: This is the wheel diameter in inches. It specifies the diameter of the wheel that the tyre is designed to fit.

Key Metrics in Tyre Fitment

When you change tyre sizes, several critical dimensions are affected:

  • Sidewall Height: This is the vertical distance from the wheel rim to the top of the tread. It directly impacts the overall comfort and handling characteristics. A lower aspect ratio means a shorter sidewall.
  • Overall Diameter: This is the total height of the tyre from the ground to the top. It's a crucial factor for speedometer accuracy, ground clearance, and potential rubbing issues within the wheel well.
  • Circumference: The distance covered by the tyre in one complete rotation. Like overall diameter, it directly influences how many rotations the tyre makes per mile/kilometer, thus affecting speedometer readings.

The Impact on Your Speedometer

Your vehicle's speedometer is calibrated based on the original equipment (OE) tyre's overall diameter and circumference. If you install tyres with a different overall diameter, your speedometer will no longer be accurate:

  • Larger Diameter Tyres: If your new tyres have a larger overall diameter than the original ones, your speedometer will read lower than your actual speed. For example, if your speedometer shows 60 mph, you might actually be traveling at 62 mph.
  • Smaller Diameter Tyres: If your new tyres have a smaller overall diameter, your speedometer will read higher than your actual speed. If your speedometer shows 60 mph, you might actually be traveling at 58 mph.

Most experts recommend keeping the speedometer error within +/- 3% to avoid legal issues (speeding tickets) and ensure safety. Significant discrepancies can also affect odometer readings, transmission shift points, and ABS/traction control systems.

Why Use a Tyre Fitment Calculator?

This calculator helps you:

  1. Verify Compatibility: Quickly see if a desired tyre size is dimensionally close to your original setup.
  2. Assess Speedometer Accuracy: Understand the exact percentage of speedometer error you can expect.
  3. Prevent Rubbing: While not a guarantee, a significant increase in overall diameter or width can indicate potential rubbing issues with fenders or suspension components.
  4. Plan for Performance: Enthusiasts often change tyre sizes for aesthetic or performance reasons (e.g., wider tyres for grip, lower profile for sharper handling). The calculator helps quantify the changes.

Example Scenario:

Let's say your original tyres are 205/55R16, and you're considering upgrading to 225/45R17.

  • Original (205/55R16):
    • Width: 205 mm
    • Sidewall Height: 205 * 0.55 = 112.75 mm
    • Wheel Diameter: 16 inches = 406.4 mm
    • Overall Diameter: (2 * 112.75) + 406.4 = 631.9 mm
  • New (225/45R17):
    • Width: 225 mm
    • Sidewall Height: 225 * 0.45 = 101.25 mm
    • Wheel Diameter: 17 inches = 431.8 mm
    • Overall Diameter: (2 * 101.25) + 431.8 = 634.3 mm

In this example, the new tyre has an overall diameter of 634.3 mm, while the original was 631.9 mm. This is a difference of +2.4 mm, resulting in a diameter increase of approximately 0.38%. This small change means that when your speedometer reads 60 mph, your actual speed would be around 60.23 mph – a negligible difference that is well within acceptable limits.

Always consult your vehicle manufacturer's recommendations or a professional tyre specialist before making significant changes to your tyre size.

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