Understanding wheel fitment is crucial for both performance and aesthetics. This calculator helps you compare your current wheel and tire setup with a potential new one, providing key measurements like changes in outer and inner clearance, and backspacing.
Proper fitment ensures your wheels don't rub against fenders or suspension components, and helps maintain optimal handling characteristics. Use this tool to make informed decisions before purchasing new wheels.
Current Wheel & Tire Specifications
New Wheel & Tire Specifications
Fitment Analysis Results
Current Wheel Backspacing:
New Wheel Backspacing:
Change in Outer Position (Fender Clearance):
Change in Inner Position (Suspension/Brake Clearance):
Understanding Wheel Fitment Terms
Wheel Width: Measured in inches, this is the distance between the inner edges of the wheel flanges where the tire bead seats. It's crucial for determining tire compatibility and overall wheel stance.
Offset (ET): Measured in millimeters, offset is the distance from the wheel's mounting surface to the true centerline of the wheel.
Positive Offset: The mounting surface is towards the front (outboard) of the wheel's centerline. This pulls the wheel further into the fender well.
Negative Offset: The mounting surface is towards the back (inboard) of the wheel's centerline. This pushes the wheel further out from the fender well.
Zero Offset: The mounting surface is exactly at the wheel's centerline.
Backspacing: Measured in inches, backspacing is the distance from the mounting surface to the back (inner) edge of the wheel. It directly impacts how close the wheel sits to suspension components, brakes, and the frame.
Outer Position (Fender Clearance): Refers to how far the wheel's outer edge sits relative to the fender. A positive change means the wheel moves further out, potentially causing fender rub. A negative change means it moves further in.
Inner Position (Suspension/Brake Clearance): Refers to how far the wheel's inner edge sits relative to suspension components, brake calipers, or the frame. A positive change means the wheel moves further in, increasing the risk of rubbing. A negative change means it moves further out.
How to Use This Calculator
Measure Your Current Setup: Find the width (usually stamped on the wheel or tire) and offset (often stamped on the back of the wheel, e.g., "ET35") of your existing wheels.
Input Current Specs: Enter these values into the "Current Wheel & Tire Specifications" fields.
Input New Specs: Enter the width and offset of the wheels you are considering into the "New Wheel & Tire Specifications" fields.
Calculate: Click the "Calculate Fitment" button.
Interpret Results:
Change in Outer Position: A positive number means the new wheel will stick out further towards the fender. A negative number means it will sit further in.
Change in Inner Position: A positive number means the new wheel will sit closer to the suspension/brakes. A negative number means it will move further away.
Consider your vehicle's existing clearances. Even a few millimeters can make a difference!
function calculateWheelFitment() {
var currentWheelWidth = parseFloat(document.getElementById('currentWheelWidth').value);
var currentWheelOffset = parseFloat(document.getElementById('currentWheelOffset').value);
var newWheelWidth = parseFloat(document.getElementById('newWheelWidth').value);
var newWheelOffset = parseFloat(document.getElementById('newWheelOffset').value);
var resultDiv = document.getElementById('fitmentResult');
var currentBackspacingSpan = document.getElementById('currentBackspacing');
var newBackspacingSpan = document.getElementById('newBackspacing');
var changeOuterSpan = document.getElementById('changeOuter');
var changeInnerSpan = document.getElementById('changeInner');
var fitmentSummaryP = document.getElementById('fitmentSummary');
if (isNaN(currentWheelWidth) || isNaN(currentWheelOffset) || isNaN(newWheelWidth) || isNaN(newWheelOffset) ||
currentWheelWidth <= 0 || newWheelWidth 0) {
summaryText += " The new wheel will extend further out by " + changeOuter.toFixed(2) + " mm, potentially closer to the fender.";
} else if (changeOuter 0) {
summaryText += " The new wheel will sit closer to the suspension/brakes by " + changeInner.toFixed(2) + " mm, increasing risk of rubbing.";
} else if (changeInner < 0) {
summaryText += " The new wheel will move further away from the suspension/brakes by " + Math.abs(changeInner).toFixed(2) + " mm, increasing clearance.";
} else {
summaryText += " The new wheel's inner edge will be in the same position relative to suspension/brakes.";
}
fitmentSummaryP.textContent = summaryText;
}