Wheel Fitment Calculator by Car

Wheel & Tire Fitment Calculator

Use this calculator to compare your current wheel and tire setup with a potential new setup. Understanding how changes in wheel diameter, width, offset, and tire dimensions affect fitment is crucial for avoiding rubbing issues, maintaining proper suspension geometry, and ensuring speedometer accuracy. Input your current and desired wheel and tire specifications below to see how the new setup will compare.

Current Setup

New Setup

Calculation Results

Enter values and click 'Calculate Fitment' to see results.

Understanding Wheel & Tire Fitment

Wheel and tire fitment refers to how a wheel and tire assembly sits within your vehicle's fender wells and interacts with suspension components. Proper fitment is essential for safety, performance, and aesthetics.

Key Terms Explained:

  • Wheel Diameter (inches): The diameter of the wheel, measured from bead seat to bead seat. Larger diameters often mean lower profile tires.
  • Wheel Width (inches): The width of the wheel, measured from bead seat to bead seat. A wider wheel can accommodate a wider tire.
  • Wheel Offset (mm): The distance from the wheel's hub mounting surface to the centerline of the wheel.
    • Positive Offset: The mounting surface is towards the front (outboard) side of the wheel. This pulls 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.
    • Zero Offset: The mounting surface is exactly at the centerline of the wheel.
  • Tire Width (mm): The width of the tire's tread section in millimeters.
  • Tire Aspect Ratio (%): The height of the tire's sidewall as a percentage of its width. For example, a 225/45R17 tire has a sidewall height that is 45% of 225mm.
  • Tire Diameter: The overall height of the tire when mounted and inflated. Changes in tire diameter directly affect speedometer readings and ground clearance.
  • Wheel Backspacing: The distance from the back edge of the wheel to its hub mounting surface. It's another way to measure how far the wheel sits inward.

Why Fitment Matters:

Incorrect fitment can lead to tires rubbing against fenders or suspension components, especially during turns or over bumps. It can also negatively impact handling, accelerate wear on suspension parts, and cause your speedometer to read inaccurately. This calculator helps you predict these changes before making a purchase.

function calculateFitment() { // Get current setup values var currentWheelDiameter = parseFloat(document.getElementById('currentWheelDiameter').value); var currentWheelWidth = parseFloat(document.getElementById('currentWheelWidth').value); var currentWheelOffset = parseFloat(document.getElementById('currentWheelOffset').value); var currentTireWidth = parseFloat(document.getElementById('currentTireWidth').value); var currentTireAspect = parseFloat(document.getElementById('currentTireAspect').value); // Get new setup values var newWheelDiameter = parseFloat(document.getElementById('newWheelDiameter').value); var newWheelWidth = parseFloat(document.getElementById('newWheelWidth').value); var newWheelOffset = parseFloat(document.getElementById('newWheelOffset').value); var newTireWidth = parseFloat(document.getElementById('newTireWidth').value); var newTireAspect = parseFloat(document.getElementById('newTireAspect').value); var resultDiv = document.getElementById('result'); var errorMessage = "; // Validate inputs if (isNaN(currentWheelDiameter) || currentWheelDiameter <= 0 || isNaN(currentWheelWidth) || currentWheelWidth <= 0 || isNaN(currentWheelOffset) || isNaN(currentTireWidth) || currentTireWidth <= 0 || isNaN(currentTireAspect) || currentTireAspect <= 0 || isNaN(newWheelDiameter) || newWheelDiameter <= 0 || isNaN(newWheelWidth) || newWheelWidth <= 0 || isNaN(newWheelOffset) || isNaN(newTireWidth) || newTireWidth <= 0 || isNaN(newTireAspect) || newTireAspect <= 0) { errorMessage = 'Please enter valid positive numbers for all fields.'; resultDiv.innerHTML = '

Calculation Results

' + errorMessage; return; } // — Current Setup Calculations — var currentTireSidewallHeight_mm = currentTireWidth * (currentTireAspect / 100); var currentTireDiameter_inches = (currentTireSidewallHeight_mm * 2 / 25.4) + currentWheelDiameter; // Wheel edge positions relative to hub mounting surface var currentWheelOuterEdge_mm = (currentWheelWidth / 2 * 25.4) – currentWheelOffset; var currentWheelInnerEdge_mm = (currentWheelWidth / 2 * 25.4) + currentWheelOffset; // Tire edge positions relative to hub mounting surface (assuming tire centered on wheel centerline) var currentTireOuterEdge_mm = (currentTireWidth / 2) – currentWheelOffset; var currentTireInnerEdge_mm = (currentTireWidth / 2) + currentWheelOffset; // — New Setup Calculations — var newTireSidewallHeight_mm = newTireWidth * (newTireAspect / 100); var newTireDiameter_inches = (newTireSidewallHeight_mm * 2 / 25.4) + newWheelDiameter; // Wheel edge positions relative to hub mounting surface var newWheelOuterEdge_mm = (newWheelWidth / 2 * 25.4) – newWheelOffset; var newWheelInnerEdge_mm = (newWheelWidth / 2 * 25.4) + newWheelOffset; // Tire edge positions relative to hub mounting surface (assuming tire centered on wheel centerline) var newTireOuterEdge_mm = (newTireWidth / 2) – newWheelOffset; var newTireInnerEdge_mm = (newTireWidth / 2) + newWheelOffset; // — Comparison Calculations — var changeTireDiameter_inches = newTireDiameter_inches – currentTireDiameter_inches; var changeTireDiameter_mm = changeTireDiameter_inches * 25.4; var changeSidewallHeight_mm = newTireSidewallHeight_mm – currentTireSidewallHeight_mm; var changeWheelOuterEdge_mm = newWheelOuterEdge_mm – currentWheelOuterEdge_mm; var changeWheelInnerEdge_mm = newWheelInnerEdge_mm – currentWheelInnerEdge_mm; var changeTireOuterEdge_mm = newTireOuterEdge_mm – currentTireOuterEdge_mm; var changeTireInnerEdge_mm = newTireInnerEdge_mm – currentTireInnerEdge_mm; var speedoError_percent = 0; if (currentTireDiameter_inches !== 0) { speedoError_percent = ((newTireDiameter_inches – currentTireDiameter_inches) / currentTireDiameter_inches) * 100; } // — Format Results — var resultsHtml = '

Calculation Results

'; resultsHtml += ''; resultsHtml += ''; resultsHtml += ''; resultsHtml += ''; resultsHtml += ''; resultsHtml += ''; resultsHtml += ''; resultsHtml += ''; resultsHtml += ''; resultsHtml += '
MetricCurrentNewChange
Tire Sidewall Height' + currentTireSidewallHeight_mm.toFixed(2) + ' mm' + newTireSidewallHeight_mm.toFixed(2) + ' mm' + (changeSidewallHeight_mm > 0 ? '+' : ") + changeSidewallHeight_mm.toFixed(2) + ' mm
Overall Tire Diameter' + currentTireDiameter_inches.toFixed(2) + ' inches (' + (currentTireDiameter_inches * 25.4).toFixed(2) + ' mm)' + newTireDiameter_inches.toFixed(2) + ' inches (' + (newTireDiameter_inches * 25.4).toFixed(2) + ' mm)' + (changeTireDiameter_inches > 0 ? '+' : ") + changeTireDiameter_inches.toFixed(2) + ' inches (' + (changeTireDiameter_mm > 0 ? '+' : ") + changeTireDiameter_mm.toFixed(2) + ' mm)
Wheel Outer Edge Position' + currentWheelOuterEdge_mm.toFixed(2) + ' mm' + newWheelOuterEdge_mm.toFixed(2) + ' mm' + (changeWheelOuterEdge_mm > 0 ? '+' : ") + changeWheelOuterEdge_mm.toFixed(2) + ' mm ' + (changeWheelOuterEdge_mm > 0 ? '(Further Out)' : (changeWheelOuterEdge_mm < 0 ? '(Further In)' : '(No Change)')) + '
Wheel Inner Edge Position' + currentWheelInnerEdge_mm.toFixed(2) + ' mm' + newWheelInnerEdge_mm.toFixed(2) + ' mm' + (changeWheelInnerEdge_mm > 0 ? '+' : ") + changeWheelInnerEdge_mm.toFixed(2) + ' mm ' + (changeWheelInnerEdge_mm > 0 ? '(Closer to Suspension)' : (changeWheelInnerEdge_mm < 0 ? '(Further from Suspension)' : '(No Change)')) + '
Tire Outer Edge Position' + currentTireOuterEdge_mm.toFixed(2) + ' mm' + newTireOuterEdge_mm.toFixed(2) + ' mm' + (changeTireOuterEdge_mm > 0 ? '+' : ") + changeTireOuterEdge_mm.toFixed(2) + ' mm ' + (changeTireOuterEdge_mm > 0 ? '(Further Out)' : (changeTireOuterEdge_mm < 0 ? '(Further In)' : '(No Change)')) + '
Tire Inner Edge Position' + currentTireInnerEdge_mm.toFixed(2) + ' mm' + newTireInnerEdge_mm.toFixed(2) + ' mm' + (changeTireInnerEdge_mm > 0 ? '+' : ") + changeTireInnerEdge_mm.toFixed(2) + ' mm ' + (changeTireInnerEdge_mm > 0 ? '(Closer to Suspension)' : (changeTireInnerEdge_mm < 0 ? '(Further from Suspension)' : '(No Change)')) + '
Speedometer ErrorN/A' + speedoError_percent.toFixed(2) + '% ' + (speedoError_percent > 0 ? '(Slower than Actual)' : (speedoError_percent < 0 ? '(Faster than Actual)' : '(No Change)')) + '
'; resultsHtml += 'Note: Positive values for "Further Out" mean the component moves away from the vehicle\'s center. Positive values for "Closer to Suspension" mean the component moves towards the vehicle\'s center.'; resultDiv.innerHTML = resultsHtml; }

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