Choosing the right wheels and tires for your vehicle is crucial for performance, safety, and aesthetics. This calculator helps you compare your current wheel and tire setup with a potential new setup to understand how they will fit. Proper fitment ensures that your tires do not rub against suspension components, fenders, or other parts of your car, especially when turning or compressing the suspension.
The Math Behind the Fitment
The core of wheel fitment calculation involves understanding the overall diameter of the tire and the position of the wheel relative to the vehicle's hub.
Tire Diameter Calculation:
The total diameter of a tire is determined by the wheel diameter plus the height of the tire's sidewall on both sides.
The sidewall height is calculated from the tire width and aspect ratio:
We convert the wheel diameter from inches to millimeters by multiplying by 25.4 (since 1 inch = 25.4 mm).
Wheel Offset and Clearance:
Wheel offset is the distance from the wheel's mounting surface to the centerline of the wheel.
– A positive offset means the mounting surface is closer to the wheel's outer face.
– A negative offset means the mounting surface is closer to the wheel's inner face.
When comparing two wheel setups, the difference in offset significantly impacts whether the wheel will sit further in or out relative to the suspension and fender. A larger positive offset will push the wheel inwards, while a larger negative offset will push it outwards. The difference between the new and old offset (New Offset – Original Offset) tells you how much further in or out the new wheel will sit.
The difference in overall tire diameter indicates how much the tire's outer edge will be closer to or further from the fender or suspension components.
Interpreting the Results
The calculator provides the differences in overall tire diameter and wheel offset.
Tire Diameter Difference: A positive number means the new tire is larger in diameter. A negative number means it's smaller. Significant changes can affect speedometer readings, odometer accuracy, and clearance.
Offset Difference: A positive number means the new wheel will sit further inwards (towards the suspension). A negative number means it will sit further outwards (towards the fender).
It's crucial to consider these differences in conjunction with your vehicle's specific clearance limitations. Generally, keeping the overall tire diameter change within +/- 3% and the offset change within +/- 10mm to 15mm is a good starting point, but always verify with your vehicle's manufacturer specifications or a professional installer.
Disclaimer
This calculator is for informational purposes only. Always consult your vehicle's owner's manual, wheel manufacturer's specifications, and a qualified professional installer before making any modifications to your vehicle's wheels or tires. Incorrect fitment can lead to serious safety issues.
function calculateFitment() {
var originalWheelDiameter = parseFloat(document.getElementById("originalWheelDiameter").value);
var originalTireWidth = parseFloat(document.getElementById("originalTireWidth").value);
var originalTireAspectRatio = parseFloat(document.getElementById("originalTireAspectRatio").value);
var originalWheelOffset = parseFloat(document.getElementById("originalWheelOffset").value);
var newWheelDiameter = parseFloat(document.getElementById("newWheelDiameter").value);
var newTireWidth = parseFloat(document.getElementById("newTireWidth").value);
var newTireAspectRatio = parseFloat(document.getElementById("newTireAspectRatio").value);
var newWheelOffset = parseFloat(document.getElementById("newWheelOffset").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous results
// Input validation
if (isNaN(originalWheelDiameter) || isNaN(originalTireWidth) || isNaN(originalTireAspectRatio) || isNaN(originalWheelOffset) ||
isNaN(newWheelDiameter) || isNaN(newTireWidth) || isNaN(newTireAspectRatio) || isNaN(newWheelOffset)) {
resultDiv.innerHTML = "Please enter valid numbers for all fields.";
return;
}
if (originalWheelDiameter <= 0 || originalTireWidth <= 0 || originalTireAspectRatio <= 0 || newWheelDiameter <= 0 || newTireWidth <= 0 || newTireAspectRatio <= 0) {
resultDiv.innerHTML = "Diameter, width, and aspect ratio must be positive values.";
return;
}
if (newWheelOffset 100 || originalWheelOffset 100) {
// Basic check for extreme offsets, can be refined
resultDiv.innerHTML = "Offset values seem unusually large. Please double-check.";
return;
}
// Calculations
// Tire Sidewall Height in mm = (Tire Width (mm) * Tire Aspect Ratio (%)) / 100
var originalSidewallHeight = (originalTireWidth * originalTireAspectRatio) / 100;
var newSidewallHeight = (newTireWidth * newTireAspectRatio) / 100;
// Tire Diameter in mm = (Wheel Diameter (inches) * 25.4) + (2 * Sidewall Height (mm))
var originalTireDiameterMM = (originalWheelDiameter * 25.4) + (2 * originalSidewallHeight);
var newTireDiameterMM = (newWheelDiameter * 25.4) + (2 * newSidewallHeight);
// Differences
var tireDiameterDifferenceMM = newTireDiameterMM – originalTireDiameterMM;
var offsetDifferenceMM = newWheelOffset – originalWheelOffset;
// Percentage change in tire diameter for context
var tireDiameterDifferencePercent = (tireDiameterDifferenceMM / originalTireDiameterMM) * 100;
// Display Results
var resultHTML = '