Fitment Calculator

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Wheel Fitment Calculator

Use this calculator to determine key dimensions for your wheel and tire setup, including backspacing, overall tire diameter, and overall tire width. Understanding these measurements is crucial for ensuring proper fitment and avoiding rubbing issues.

Wheel Specifications

Tire Specifications

Calculated Fitment Details

Enter your wheel and tire specifications above and click "Calculate Fitment" to see the results.

function calculateFitment() { var wheelDiameter = parseFloat(document.getElementById('wheelDiameter').value); var wheelWidth = parseFloat(document.getElementById('wheelWidth').value); var offset = parseFloat(document.getElementById('offset').value); var tireWidth = parseFloat(document.getElementById('tireWidth').value); var aspectRatio = parseFloat(document.getElementById('aspectRatio').value); var resultDiv = document.getElementById('fitmentResult'); if (isNaN(wheelDiameter) || isNaN(wheelWidth) || isNaN(offset) || isNaN(tireWidth) || isNaN(aspectRatio) || wheelDiameter <= 0 || wheelWidth <= 0 || tireWidth <= 0 || aspectRatio <= 0) { resultDiv.innerHTML = 'Please enter valid positive numbers for all fields.'; return; } // Calculations // 1 inch = 25.4 mm // Backspacing (inches) = (Wheel Width / 2) + (Offset / 25.4) var backspacing = (wheelWidth / 2) + (offset / 25.4); // Sidewall Height (mm) = (Tire Section Width * Aspect Ratio / 100) var sidewallHeightMM = (tireWidth * aspectRatio / 100); // Sidewall Height (inches) = Sidewall Height (mm) / 25.4 var sidewallHeightInches = sidewallHeightMM / 25.4; // Overall Tire Diameter (inches) = Wheel Diameter + (2 * Sidewall Height (inches)) var overallTireDiameter = wheelDiameter + (2 * sidewallHeightInches); // Overall Tire Width (inches) = Tire Section Width (mm) / 25.4 var overallTireWidth = tireWidth / 25.4; // Inner Wheel Position (from mounting surface, inches) = Backspacing var innerWheelPosition = backspacing; // Outer Wheel Position (from mounting surface, inches) = (Wheel Width / 2) – (Offset / 25.4) var outerWheelPosition = (wheelWidth / 2) – (offset / 25.4); resultDiv.innerHTML = 'Calculated Backspacing: ' + backspacing.toFixed(2) + ' inches' + 'Overall Tire Diameter: ' + overallTireDiameter.toFixed(2) + ' inches' + 'Overall Tire Width: ' + overallTireWidth.toFixed(2) + ' inches' + 'Inner Wheel Position (from hub mounting surface): ' + innerWheelPosition.toFixed(2) + ' inches' + 'Outer Wheel Position (from hub mounting surface): ' + outerWheelPosition.toFixed(2) + ' inches'; } .wheel-fitment-calculator-container { font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif; background-color: #f9f9f9; padding: 25px; border-radius: 10px; box-shadow: 0 4px 12px rgba(0, 0, 0, 0.1); max-width: 700px; margin: 30px auto; border: 1px solid #e0e0e0; } .wheel-fitment-calculator-container h2 { color: #2c3e50; text-align: center; margin-bottom: 20px; font-size: 28px; } .wheel-fitment-calculator-container h3 { color: #34495e; margin-top: 25px; margin-bottom: 15px; font-size: 22px; border-bottom: 2px solid #ececec; padding-bottom: 8px; } .wheel-fitment-calculator-container p { color: #555; line-height: 1.6; margin-bottom: 10px; } .calculator-inputs label { display: block; margin-bottom: 8px; margin-top: 15px; color: #333; font-weight: bold; font-size: 15px; } .calculator-inputs input[type="number"] { width: calc(100% – 22px); padding: 12px; margin-bottom: 15px; border: 1px solid #ccc; border-radius: 6px; font-size: 16px; box-sizing: border-box; transition: border-color 0.3s ease; } .calculator-inputs input[type="number"]:focus { border-color: #007bff; outline: none; box-shadow: 0 0 5px rgba(0, 123, 255, 0.2); } .calculator-inputs button { background-color: #007bff; color: white; padding: 14px 25px; border: none; border-radius: 6px; cursor: pointer; font-size: 18px; margin-top: 20px; width: 100%; transition: background-color 0.3s ease, transform 0.2s ease; } .calculator-inputs button:hover { background-color: #0056b3; transform: translateY(-2px); } .calculator-results { margin-top: 30px; background-color: #e9f7ff; padding: 20px; border-radius: 8px; border: 1px solid #cceeff; } .calculator-results p { font-size: 16px; color: #333; margin-bottom: 8px; } .calculator-results p strong { color: #0056b3; }

Understanding Wheel Fitment

Wheel fitment refers to how a wheel and tire assembly sits within the wheel well of a vehicle. Proper fitment is crucial for safety, performance, and aesthetics. Incorrect fitment can lead to issues like tire rubbing, premature wear, compromised handling, and even damage to suspension components.

Key Fitment Terms Explained:

  • Wheel Diameter (inches): This is the measurement of the wheel from one side to the other, through the center. Common diameters range from 15 to 24 inches.
  • Wheel Width (inches): This measures the distance between the inner and outer bead seats of the wheel. It's important for tire compatibility and overall stance.
  • Wheel Offset (mm): Offset is the distance from the wheel's hub mounting surface to the true centerline of the wheel.
    • Positive Offset: The mounting surface is towards the front (outboard) side of the wheel. This pushes the wheel further into the fender.
    • Negative Offset: The mounting surface is towards the back (inboard) side of the wheel. This pushes the wheel further out from the fender, creating a "deeper dish" look.
    • Zero Offset: The mounting surface is exactly at the centerline of the wheel.
  • Tire Section Width (mm): This is the width of the tire from sidewall to sidewall, measured in millimeters. For example, in a 245/40R18 tire, '245' is the section width.
  • Tire Aspect Ratio (%): Also known as the profile, this is the height of the tire's sidewall expressed as a percentage of the tire's section width. In 245/40R18, '40' means the sidewall height is 40% of 245mm.

What the Calculator Determines:

  • Backspacing (inches): This is the distance from the wheel's hub mounting surface to the back edge of the wheel. It's a critical measurement for ensuring the wheel clears suspension components and inner fender liners. A higher backspacing value means the wheel sits further inward.
  • Overall Tire Diameter (inches): The total height of the tire when mounted on the wheel. This affects speedometer accuracy, gearing, and fender clearance.
  • Overall Tire Width (inches): The total width of the tire, which is important for fender clearance and avoiding rubbing on steering components.
  • Inner/Outer Wheel Position (from hub mounting surface): These values help visualize how much the wheel extends inward or outward from the vehicle's hub, aiding in clearance checks.

Why is this important?

Using this calculator helps you predict how a new wheel and tire combination will fit your vehicle before making a purchase. By understanding backspacing, overall diameter, and width, you can avoid common fitment issues like:

  • Tires rubbing against fenders during turns or suspension compression.
  • Wheels hitting brake calipers or suspension arms.
  • An undesirable "tucked" or "poked" look that doesn't match your aesthetic goals.

Always double-check your vehicle's specific clearance requirements and consider professional advice when making significant changes to wheel and tire sizes.

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