Use this calculator to compare two tire sizes and understand the impact on your vehicle's speedometer, ride height, and overall diameter.
Original Tire Specifications
New Tire Specifications
Understanding Tire Sizes and Conversions
Changing your vehicle's tire size can have significant effects on its performance, appearance, and even safety. A tire conversion calculator helps you compare different tire dimensions and predict these changes before you make a purchase.
How to Read a Tire Size
Tire sizes are typically displayed in a format like 205/55R16:
205: This is the tire's section width in millimeters. It measures the width of the tire from sidewall to sidewall.
55: This is the aspect ratio, expressed as a percentage. It represents the sidewall height as a percentage of the tire's width. In this example, the sidewall height is 55% of 205mm.
R: Indicates a radial construction tire, which is the most common type today.
16: This is the rim or wheel diameter in inches.
Why Use a Tire Conversion Calculator?
When considering new tires or wheels, especially for upgrades or modifications, it's crucial to understand how the new size compares to your original equipment. Key reasons include:
Speedometer Accuracy: A change in overall tire diameter directly affects your speedometer reading. If the new tire is larger, your speedometer will read slower than your actual speed. If it's smaller, it will read faster.
Fender Clearance: Larger tires might rub against your vehicle's fenders or suspension components, especially during turns or over bumps.
Ride Comfort and Handling: A lower aspect ratio (shorter sidewall) generally means a firmer ride and potentially sharper handling, while a higher aspect ratio offers more cushioning.
Braking Performance: Significant changes in tire diameter can affect braking dynamics.
Aesthetics: Many people change tire sizes for a specific look, such as a larger wheel diameter with a lower profile tire.
Key Metrics Calculated
Our calculator provides the following crucial comparisons:
Tire Diameter: The total height of the tire from the ground to the top. This is the primary factor affecting speedometer accuracy.
Sidewall Height: The height of the rubber section between the rim and the tread. This impacts ride comfort and appearance.
Tire Circumference: The distance covered by one full rotation of the tire. This is directly related to speedometer readings.
Speedometer Error: The percentage difference in speed reading at a given actual speed (e.g., 60 mph).
General Guidelines for Tire Conversion
While there's no universal rule, most experts recommend keeping the overall tire diameter change within +/- 3% of the original diameter. Exceeding this range can lead to significant speedometer errors, potential rubbing issues, and adverse effects on vehicle safety systems (like ABS and traction control) that rely on accurate wheel speed data.
Example Calculation:
Let's say your original tires are 205/55R16 and you're considering 225/45R17.
Original: 205/55R16
Width: 205 mm
Aspect Ratio: 55%
Rim Diameter: 16 inches
New: 225/45R17
Width: 225 mm
Aspect Ratio: 45%
Rim Diameter: 17 inches
Inputting these values into the calculator would show you the exact differences in diameter, sidewall, and circumference, along with the speedometer error. For instance, if the new tire is slightly larger, the calculator might tell you that when your speedometer reads 60 mph, your actual speed is 60.5 mph.
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function calculateTireConversion() {
// Get original tire inputs
var originalWidth = parseFloat(document.getElementById('originalWidth').value);
var originalAspect = parseFloat(document.getElementById('originalAspect').value);
var originalRim = parseFloat(document.getElementById('originalRim').value);
// Get new tire inputs
var newWidth = parseFloat(document.getElementById('newWidth').value);
var newAspect = parseFloat(document.getElementById('newAspect').value);
var newRim = parseFloat(document.getElementById('newRim').value);
var resultDiv = document.getElementById('tireConversionResult');
resultDiv.innerHTML = "; // Clear previous results
// Validate inputs
if (isNaN(originalWidth) || isNaN(originalAspect) || isNaN(originalRim) ||
isNaN(newWidth) || isNaN(newAspect) || isNaN(newRim) ||
originalWidth <= 0 || originalAspect <= 0 || originalRim <= 0 ||
newWidth <= 0 || newAspect <= 0 || newRim <= 0) {
resultDiv.innerHTML = 'Please enter valid positive numbers for all tire specifications.';
return;
}
// — Calculations for Original Tire —
var originalSidewallMM = originalWidth * (originalAspect / 100);
var originalSidewallInches = originalSidewallMM / 25.4; // Convert mm to inches
var originalDiameterInches = (2 * originalSidewallInches) + originalRim;
var originalCircumferenceInches = originalDiameterInches * Math.PI;
// — Calculations for New Tire —
var newSidewallMM = newWidth * (newAspect / 100);
var newSidewallInches = newSidewallMM / 25.4; // Convert mm to inches
var newDiameterInches = (2 * newSidewallInches) + newRim;
var newCircumferenceInches = newDiameterInches * Math.PI;
// — Differences —
var diameterDifferenceInches = newDiameterInches – originalDiameterInches;
var diameterDifferencePercent = (diameterDifferenceInches / originalDiameterInches) * 100;
var sidewallDifferenceMM = newSidewallMM – originalSidewallMM;
var sidewallDifferencePercent = (sidewallDifferenceMM / originalSidewallMM) * 100;
var circumferenceDifferenceInches = newCircumferenceInches – originalCircumferenceInches;
var circumferenceDifferencePercent = (circumferenceDifferenceInches / originalCircumferenceInches) * 100;
// — Speedometer Impact —
// What the speedometer will read if actual speed is 60 mph
var speedometerReadingAt60MPHActual = 60 * (originalDiameterInches / newDiameterInches);
// — Display Results —
var resultsHTML = '
';
if (diameterDifferencePercent > 0.1) { // New tire is significantly larger
resultsHTML += 'When your actual speed is 60 mph, your speedometer will read approximately ' + speedometerReadingAt60MPHActual.toFixed(2) + ' mph.';
resultsHTML += 'This means your speedometer will under-read by about ' + Math.abs(diameterDifferencePercent).toFixed(2) + '%.';
} else if (diameterDifferencePercent < -0.1) { // New tire is significantly smaller
resultsHTML += 'When your actual speed is 60 mph, your speedometer will read approximately ' + speedometerReadingAt60MPHActual.toFixed(2) + ' mph.';
resultsHTML += 'This means your speedometer will over-read by about ' + Math.abs(diameterDifferencePercent).toFixed(2) + '%.';
} else {
resultsHTML += 'No significant speedometer error (less than 0.1% difference).';
}
if (Math.abs(diameterDifferencePercent) > 3) {
resultsHTML += 'Warning: The overall diameter difference of ' + diameterDifferencePercent.toFixed(2) + '% is outside the recommended +/- 3% range. This may cause significant speedometer errors, potential rubbing, and affect vehicle safety systems.';
} else {
resultsHTML += 'The overall diameter difference of ' + diameterDifferencePercent.toFixed(2) + '% is within the recommended +/- 3% range.';
}
resultDiv.innerHTML = resultsHTML;
}
function clearTireConversion() {
document.getElementById('originalWidth').value = '205';
document.getElementById('originalAspect').value = '55';
document.getElementById('originalRim').value = '16';
document.getElementById('newWidth').value = '225';
document.getElementById('newAspect').value = '45';
document.getElementById('newRim').value = '17';
document.getElementById('tireConversionResult').innerHTML = ";
}